Phrasal verbs belong to common, informal everyday language. You definitely need to know them if you want to use English naturally, because using the formal equivalent often sounds awkward.
We understand the meaning of phrasal verbs based on context - there's usually something else in the sentence which helps with them meaning, especially for phrasal verbs which can have multiple meanings (eg. look up, make out, break off...). Also, phrasal verbs tend to give us a little more information than just the basic verb.
Were you asking about specific examples? Are there any which confuse you?