One of the different orders in which you can place a given set of things.

If you had two words - 'cat' and 'sat', then the only phrases you could write are: 'cat sat' or 'sat cat'.

If you had three words - cat, sat and mat then the phrases you could write are:

'cat sat mat'
'cat mat sat'
'sat mat cat'
'sat cat mat'
'mat cat sat'
'mat sat cat'

Two words had two permutations, while three words had six rearrangements.