Some verbs are considered weak because they don't help the reader picture the action of a sentence. To explain, weak verbs usually just describe a noun's state of being rather than what it is doing, and seeing what someone or something is doing is what makes a sentence interesting to read. As the linked handout explains, some examples of weak verbs are is, are, was, were, has, have, had, will be, have been, has been, had been, could have been, should have been, and would have been. Notice how each of those words is vague: you can't really picture anything happening in your mind. However, with strong verbs, you can imagine the described action because they have a specific meaning. Some examples of strong verbs are words like struck, dodged, planted, burst, dove, heaved, and shrieked.
Please see the following link for more examples of strong verbs and how to identify weak ones: