Verbs are "doing" or "action" words. For example, walk, run and cycle are verbs.
All sentences need a verb. What is the verb in that sentence? NEED. All sentences NEED a verb. Without a verb, there would not be a sentence. Would this make sense, for example: "All sentences a verb"? Even if we added lots of adjectives and adverbs, we still wouldn't have a sentence: "All truly excellent and marvellous, even brilliant sentences amazingly strong and interesting verb." See? It still doesn't make sense.
Verbs tell us what the subject does or is. Which is the verb in this sentence: "The cow is contented"? "Cow" is a noun. "Contented" is an adjective describing the cow. "The" is a determiner, belonging with the noun. That leaves "is". In fact, it is important to remember that verbs are not just "doing" words, but also "being" words. The verb "to be" is one of the most useful verbs we have. So look out for "am", "are" and "is", too!
You will find verbs everywhere in English writing and speech - they positively jump out at you (that's because they're action words!).
See how well you know basic verbs by playing this quiz on the subject.