An ELT Glossary : Adverb particle

An adverb particle is an adverb used as part of a phrasal verb, which may be intransitive (eg The plane took off) or transitive - eg :

He put the plates away.  (phrasal verb : put away)
She took off her hat. (phrasal verb : take off)
They gave the money back. (phrasal verb : give back)
He sent in his application. (phrasal verb : send in)

As these examples show, with a noun as object, the adverb particle may be separated from the verb and placed after the the object - ie you can say : He sent in his application  or He sent his application in. This is optional, and the separation is less common when the object is very long - eg if the object was his application for the job that David had told him about it would be unlikely that the verb and particle would be separated.

However, when the object is a pronoun, the separation is obligatory : He sent it in - never *He sent in it.

See also : 

Multiword verbs
Prepositional Verbs or Verbs with Prepositional Phrases? What's the Difference?