Simply put, anything enclosed inside << and >> is considered by LilyPond to happen simultaneously. This can be used in any context (and any Context - see above/below). It is possible to tell LilyPond that you want two notes to happen at the same time in the same voice (yielding a chord), at the same time on the same staff (yielding polyphony), and so on. Moreover, any score with multiple staves will use << and >> to tell LilyPond that the parts should begin at the same time, and creative use of << and >> is one of the keys to advanced notation.

It is not important to understand simultaneity at first. By observing how Frescobaldi creates scores for you, and how examples on the internet take advantage of these symbols, you will eventually understand how to use them.