A PHRASAL VERB is mainly used in spoken English and informal texts. They are made up by a verb plus a particle. The particle can change the meaning of the verb completely.
Here are some examples:
To look up.
To look down.
To look for a book that I lost in the library.
To look forward to the summer holidays.
There are no real rules to follow, you really just have to learn these phrases in context. The only real rule is that the particle either follows the verb or the subject.
Particle after the verb: Please write down what I am about to say.
Particle after the subject: Please write it down. („it“ is the subject)