For one to find out about the definitions of "generative grammar", "sandhi phenomenon", "agglutinative morphology", etc.
The only one which I have really used in that past is the Lexicon of Linguistics. There is also the Glossary of linguistic terms, which does not seem to be as complete. A brief search also revealed this Linguistics Glossary, which has very few entries.
In print, there are potentially more options, such as A Dictionary of Linguistics & Phonetics, but I imagine that these can become outdated rather quickly (which is not to say that websites do not, in practice, but they are theoretically easier to maintain).