A verb is the part of speech that indicates what something does, or what it is. It's also the basic component of the predicate of a sentence.
You may know that a verb is just another part of speech, like a noun or an adverb. The verb, however, can be in different tenses, meaning the action might have happened in the past or it might be now, or it could even be planned for the future. So the verb walk in the past tense would be walked, and in the future would be will walk.