If you just opened the XML file in Notepad++ and have no clue what it's about, you probably have not edited an XML documented yet. Let me give you a small introduction to XML, in the means it is used for MK8.

XML is basically just text, but written in a specific way (with this specific way being called the "syntax" - it has a lot of sharp arrows like < and > as you might have guessed). What it tries to solve with that is organizing game data in a hierarchy, like a folder structure with files. But how could one imagine such data?

Let's have a real world example. Imagine you have a jar in which you put an apple. In XML, you would write it like this:
[CODE]<jar>
  <apple>
  </apple>
</jar>[/CODE]
You basically opened the jar by writing jar in sharp arrows ([B]<jar>[/B]), put an apple inside with [B]<apple></apple>[/B] (since it has nothing inside of itself) and closed that jar again with [B]</jar>[/B]. It is important to remember how the sharp arrows and backslashes are used, because that's the mentioned "syntax". If you do that wrong, you cannot convert XML back to BYAML later and will get an error.

What if the apple actually had a worm inside of it?
[CODE]<jar>
  <apple>
    <worm>
    </worm>
  </apple>
</jar>[/CODE]
As you can see, you can extend this hierarchy by just writing stuff between [B]<blabla>[/B] and [B]</blabla>[/B].
In XML terms, you put in a worm [B]element[/B] into the apple element (which in turn is inside the jar element). In case you wondered about the weird alignment on each line: It's a good idea to indent stuff being inside of others, so you can visualize the hierarchy better. Just do that by spaces. If you don't do that exactly correct, it won't cause an error later on, but it might just look ugly.

There's actually a shorter way to write out things which having nothing inside them - like the worm here:
[CODE]<jar>
  <apple>
    <worm />
  </apple>
</jar>[/CODE]
Notice the different placing of the backslash in [B]<worm />[/B]. It basically means the same as the XML in the previous example.

Now, we want to give the jar a color. Damn. A color is not "content" of the jar, it is more like a description of how the jar is. In XML, we can define such descriptions with "attributes". E.g.; "color" is an attribute of the jar. It will look like this:
[CODE]<jar color="red">
  <apple>
    <worm />
  </apple>
</jar>[/CODE]
You added stuff in between the two sharp arrows of [B]<jar>[/B], making it [B]<jar color="red">[/B]. Simply said, you give your attribute a name (e.g. what it describes: [B]color[/B]), write an equals sign, and then a value (in the case of this color, [B]red[/B]) which you write in quotes.

We can apply the same logic to the apple, describing it's ripeness:
[CODE]<jar color="red">
  <apple ripeness="rotten">
    <worm />
  </apple>
</jar>[/CODE]
Yikes. That must be why there's a worm inside of it, right?

You can also have multiple attributes for the same element:
[CODE]<jar color="red" material="glass">
  <apple ripeness="rotten">
    <worm />
  </apple>
</jar>[/CODE]
Just put a space after the last attribute and add another one.

Of course you can put multiple things into the jar:
[CODE]<jar color="red" material="glass">
  <apple ripeness="rotten">
    <worm />
  </apple>
  <banana />
  <grapefruit>
    <bigasskernel />
  </grapefruit>
  <pineapple ripeness="perfect" />
</jar>[/CODE]
That filled your jar with 4 fruits in total now - though you should probably remove the yucky apple:
[CODE]<jar color="red" material="glass">
  <banana />
  <grapefruit>
    <bigasskernel />
  </grapefruit>
  <pineapple ripeness="perfect" />
</jar>[/CODE]
You did that by just deleting the apple [B]element[/B] - it's that easy!

So, how does this apply to Mario Kart 8? We know the course_muunt.byaml can be converted to XML. Instead of describing a jar in there, it describes the whole course. Since BYAML originally wasn't XML, it doesn't do that with a nice course element as you'd expect (e.g. putting everything on the course between <course> and </course>). It does so in a [B]yaml[/B] element. Just imagine that as your course.

Like the jar, a course has many different things on it - invisible paths the AI drives, gravity zones for anti gravity sections, sounds played in specific areas, and then, simply "course objects" (which span a cheering toad, a walking cow, the moon in the sky, and mostly everything else). MK8 organizes XML slightly differently. It would've written our fruit jar like this, with the intensive use of attributes rather than different elements:
[CODE]<yaml color="red" material="glass">
  <fruits>
    <fruit type="apple" ripeness="rotten" />
    <fruit type="banana" />
    <fruit type="grapefruit" />
  </fruits>
</yaml>[/CODE]
Instead of describing the color or material of a jar, it of course describes the total lap count or if headlights of cars are turned on for the course. It would look like this:
[CODE]<yaml HeadLight="1" LapNumber="3">
  <Obj>
    <value ObjId="1013" Speed="3f" />
    <value ObjId="1015" />
    <value ObjId="1018" />
  </Obj>
  <Path>
    ...single paths in here...
  </Path>
  ... much more stuff ...
</yaml>[/CODE]
You can see it uses some quite abstract attribute names and values. All the information on what is described exactly with those follows now.