I'm new to CSS and I've seen that some have two or more font choices seperated by commas? Why is that and what does it do?

I guess you are talking about the "font-family" declaration, which is the only one that takes multiple fonts as arguments.

The reason we need font-family is that not all operating systems or browsers support all fonts, so we make a list of various fonts to fall back on if the first in the list are not supported.

So, a declaration like this:
CSS

font-family: "Courier New", Courier, monospace;

means that the first font to be considered will be "Courier New", followed by "Courier", then defaulting to "monospace". Note that any time a font is composed of two or more words, it must have quotes around it.

you can also use this for special fonts. if you like a special font that you downloaded you can set it as the first font in the font-family declaration followed by a few standard fonts (arial, courier, times new roman, etc.). This way your website will be displayed as you designed it for people with that same font downloaded but will be displayed in more standardized fonts for those that don't have it.

Keep in mind, some fonts are significantly smaller or larger with the same font-size so if you do have a custom font, make sure it is close to the same size ratio of the standard fonts you chose so your site does not completely change between two fonts.

Using 'special' fonts just isn't a good idea since only a very small fraction of a site's audience are likely to have that 'special' font installed.

If you want to use a 'special' font that can be viewed by EVERYONE, then a replacement method called sIFR works extremely well.