Phrasal verbs are a group of words that combine a verb with an adverb (take up), or a preposition (look after) or both (look forward to/get on with). Together these words act like a new word and take on a completely new meaning.
Modern English, especially the spoken one, is inconceivable without phrasal verbs. However, mastering them is not an easy task to do.  There are hundreds of phrasal verbs, and most of them have multiple meanings.