Simple Definition:

In English, modal refers to a type of auxiliary verb [a verb used with a main verb to create different tenses or forms] that expresses various meanings such as ability, necessity, permission, and possibility. For example, the modal verbs ‘can,’ ‘must,’ ‘may,’ and ‘might’ express different meanings in sentences like "She can swim," "You must finish your homework," "May I borrow your pencil?" and "It might rain today."

Very Simple Definition:

In English, modal refers to special verbs that are used to show abilities, obligations [things you need to do], permissions, and possibilities. For example, the words ‘can,’ ‘must,’ ‘may,’ and ‘might’ are modals that express different meanings. Examples of sentences using modals are "She can swim," "You must finish your homework," "May I borrow your pencil?" and "It might rain today."