WHAT'S A VERB?
A verb is a word that can express an action or a state, both in a very broad sense (for example: to write, to have, to think). Without it, a sentence cannot be complete and will probably not make any sense.
Imagine you found the entry below in a diary. Obviously you know it's got something to do with Mary and the cat, but without the illegible part, which is the verb, you do not know whether Mary saw the cat, fed or kicked it (not to mention a myriad of other plausible scenarios)!
Both in English and in French, the verb is essential as it describes the action and places it on a time line (whether it belongs to the past, present or future). To refer to the place in time of an action, one uses the term 'tenses' (past, present and future).