Phrasal verbs consists of a basic verb + another word or words. The two or three words that make up a phrasal verb form a short 'phrase' - which is why we call them 'phrasal verbs'.
Phrasal Verb:
pay someone back
Meaning / Example:
1. to return money that was borrowed from a person. > You owe me money. When are you going to pay me back? > You must pay John back. You have owed him money for a long time.
2. to get even with someone [for doing something]. > I will pay her back for what she said about me. > Fred eventually will pay Mike back. He bears grudges for a long time.
Phrasal verbs, also called two-word verbs, are idiomatic expressions wherein the second element of the verb (the adverb or particle) is not necessarily predictable.