Personification is a type of figurative language which is "a figure of speech in which abstractions, animals, ideas, and inanimate objects are endowed with human form, character, traits, or sensibilities" ("eNotes Guide to Literary Terms").  It is the attribution of human qualities to something that is not human. 

An example of personification is when Pony and Johnny are on the run after Johnny kills Bob.  Pony has been nearly drowned in the fountain, and so he is soaking wet. He and Johnny go to Dally for help, and Dally gives him a shirt that is too big.

I started buttoning up the shirt. It about swallowed me. (Ch. 4)

Pony feels overwhelmed by the entire situation.  The image of the shirt swallowing him is personification because shirts are not alive and cannot really swallow a person.  Pony feels like the shirt is swallowing him because it is so big, but it also represents his state of mind where the situation is too much for him to handle.

Another example of personification is a common idiom.  The boys have to hide at an old church while they are on the run.  Johnny goes to the store and comes back with a copy of Gone with the Wind to help the boys pass the time.

THE next four or five days were the longest days I've ever spent in my life. We killed time by reading Gone with the Wind and playing poker. (Ch. 5)

You cannot literally kill time, of course; time is not alive.  That is why it is personification.  It is an idiom because it is also a common expression, or figure of speech, as well.  That means that most people understand that to kill time means to do something to pass the time.  The boys are passing the time reading the book as they are in hiding there.