What does the Fifth Petition mean?

And forgive us our trespasses, as we forgive those who trespass against us.

That is...

We pray in this petition that our Father in heaven would not look upon our sins, nor, on account of them, deny our prayer; for we are not worthy of anything we ask, neither have we deserved it; but we pray that He would give it to us out of His mercy; for we sin every day, and deserve nothing but punishment.

And we on our part will heartily forgive those who have sinned against us, and return good for evil.