I don't know if you've seen the Java tutorial on annotations, but here is the link. It's an excellent reference: Annotations in Java
A very simple example is: before annotations the compiler had to assume that you knew what you were doing if you overrode a method, such as, paint. Now with annotations most of the IDE's will give you a little added warning via a tag that says--hey, you're doing an override--and you can then do the annotation for override "@Override" and both you and the compiler know that you actually meant to override that paint method instead of it being an accident cause by a poor naming scheme in your program
The code you quote has a BeerExpert class, not a Triangle class.
Is that just a typo?
Assuming you intended to compile
>javac BeerExpert.java
you need to make a directory structure something like:
exercise/src/com/model/BeerExpert.java
exercise/bin