See this passage explained in the notes at Matthew 6:9-13.
For we also forgive ... - This is somewhat different from the expression in Matthew, though the sense is the same. The idea is, that unless we forgive others, God will not forgive us; and unless we come to him “really” forgiving all others, we cannot expect pardon. It does not mean that by forgiving others we “deserve” forgiveness ourselves, or “merit it,” but that this is a disposition or state of mind without which God cannot consistently pardon us.
Every one that is indebted to us - Every one that has “injured” us. This does not refer to pecuniary transactions, but to offences similar to those which “we” have committed against God, and for which we ask forgiveness. Besides the variations in the “expressions” in this prayer, Luke has omitted the doxology, or close, altogether; and this shows that Jesus did nor intend that we should always use just this “form,” but that it was a general direction how to pray; or, rather, that we were to pray for these “things,” though not always using the same words.