Why can must need should are called modal words what modal means thanks for answe

Why can must need should are called modal words what modal means thanks for answers?

It’s just the name given to the type of verb that is used with another verb (not a modal) to express possibility, permission, intention, etc…

If you take a sentence like: He is taking these books back to the library , the word ‘is’ is part of the auxiliary verb ‘be’ and all it tells you is that this is the present continuous tense. But if you say: He can/He must/ He may take these books back to the library, you are indicating something else which shows the speaker’s/writer’s attitude towards what he is saying/what he is thinking and that is the ‘modality’ of the statement - in other words ‘can’ indicates possibility, ‘must’ indicates necessity and ‘may’ indicates permission. All these ‘auxiliary’ verbs express this additional information and we call them ‘modals’.