I guess you are searching for a simple interpreter example but unfortunately I have to say you found a poor one. The article looks terrible to me and the project contains dozens of files. Normally the answer to your question should be in the article. Before starting to learn anything about basic and maybe interpreters you should learn how to use google. By typing in the magic keywords "simple basic interpreter" I found this as a 3rd result, its a very small interpreter that has some really good reviews, check it out if you are interested about interpreters: http://www.programmersheaven.com/download/16060/download.aspx
You should always look for small examples or ones that have really good accompanying article as explanation. The example I linked has no attached help but the interpreter source is extremely small and easy to understand. If you want to learn how to put together interpreters or compilers you will find materials on the subject with google, search and check out a lot of them and pick one that suits your level of knowledge the best.