Phrasal verbs are a verb plus a preposition. For example fall out. They are more informal than the full verb which is argue. To fall out with someone is to argue and then not be friends for a while. Another example is SPLIT UP. Verb—split. plus preposition—up. This is the informal way of saying to separate.
A Phrasal verb is a the combination of a verb and a particle (technically speaking they are not prepositions), whose meaning together is different from the meaning of the verb and the preposition used separately. They just need to be learned as a unit. At the beginning they just need to memorize them and there isn't much to explain really.