A prefix is an affix which is placed before the stem of a word.

Some languages allow multiple prefixes before a stem , for example in English the word unenforceable consists of two prefixes, a root, and a suffix:

un- en- force -able

Some prefixes change depending on the root, for example in English in- becomes im- before certain roots:

insecure, incapable, inaccurate but immovable, impatient.

In some languages, hyphens are used to show how words are broken up into different morphemes.

Reference: prefix