Verbs are words that denote actions or states of being. Verbs are arguably the most important part of speech because they denote the action or state of being to be expressed in any given sentence. It is often helpful to think of the verb as the backbone of the sentence. We can even write a grammatically correct sentence using only a verb. We can do the same with other parts of speech in spoken language, but it is not grammatically correct (and usually not very desirable either) to attempt written communication in English without a verb.
John is my best friend.
In this simple sentence, the linking verb is states John’s existence or state of being as the speaker’s best friend.
The knight swung his heavy sword at the advancing foe.
Swung here shows an action that the knight performed.
Mr. Alexander has an expensive car.
The stative verb has here indicates possession to show that Mr. Alexander is the owner of the expensive car.