"Forgive, and you will be forgiven" is a principle known long before Christ came. But in our fallen world it is usually taken to mean mutual indulgence: close your eyes to my sins, and I will close mine to yours, and we will not spoil each other's mood with mutual reproaches and claims. Of course, such an arrangement usually works only as long as the interests of one of the parties are not seriously affected, but quite often it still helps preserve decency and the appearance of normal relations. In everyday life, this is often what is called "forgiving your neighbor his sins."
But Jesus undoubtedly means something else: for He was never tolerant of any sin, although He always forgave the repentant sinner. He plainly proceeds from the fact that we are all sinners, and each of us needs, on the threshold of the Kingdom, to be forgiven those sins of which we repent and for which we grieve. But then we have no moral right to demand an accounting from others: either we begin to reckon up sins, and then there can be no question of the Kingdom, or we allow ourselves to be judged and cleansed from sin by the One who alone can do this; then the road into the Kingdom is open to us. The choice is ours.
