if you use latex in other places or get examples from other places, they'll use different symbols to indicate the start and end of an expression. here, they use . In other places, they use dollar signs, or even nothing at all. What's important to understand is how to use a symbol you saw on one site on here. Anything with a "\" (backlash) means that it is an escape sequence, and that you have to put it in between your . my favorite place to go is mathurl.com and they provide buttons you can click to get a bunch of math symbols (like systems of equations, the summation notation, etc.) and it'll show you the LaTex as well as what the text will look like once you submit it!
To get tags to display you need to put them between [noparse][/noparse] tags