Simple Definition:

Modals are a type of auxiliary verb [a type of verb used with other verbs to modify them] that express various meanings such as possibility, ability, permission, obligation, and advice. They are used to modify the main verb in a sentence and often indicate the attitude or mood of the speaker. Some common modals include can, could, may, might, must, shall, should, will, and would.

Examples:

• She can speak French fluently. (ability)
• They may arrive late tonight. (possibility)
• You should take an umbrella with you. (advice)
• He must finish his homework before he goes out. (obligation)

Very Simple Definition:

Modals are special words that help us talk about things like what we can do, what we should do, and what we must do. They are words like can, could, may, should, and must.

Examples:

• I can swim in the pool. (ability)
• You may have some cake if you want. (permission)
• We should go to bed early tonight. (advice)
• He must wear a helmet when he rides his bike. (obligation)