Jesus teaches about forgiveness.  Family. Often family members tell us the truth, it hurts, makes us angry, then we realize they were right and now is time for our action.  This is what Jesus begins to tell the disciples at verse 15:
15 Moreover if thy brother shall trespass against thee,
go and tell him his fault between thee and him alone:
if he shall hear thee, thou hast gained thy brother.
16 But if he will not hear thee,
then take with thee one or two more,
that in the mouth of two or three witnesses
every word may be established.
17 And if he shall neglect to hear them,
tell it unto the church:
but if he neglect to hear the church,
let him be unto thee as
an heathen man and a publican.
This is what Jesus tells us to do when dealing with those who sin against us.  These guidelines are for Christians, sins committed against you, and the way to resolve the conflict.  Jesus does not give us this in order to attack a person, but to help reconcile differences between believers.
    In verses 19,20 Jesus is talking about the sincere agreement, let me say this again the SINCERE AGREEMENT between two or more believers.  This does not mean 3 or 4 of us get together and say let’s be sincere about praying to win the 165 million dollar lottery.   Also, these believers need to be filled with the Holy Spirit .  This sincere agreement is more powerful than thousands praying out of “duty” or “half-heartedly”.  The Holy Spirit  HAS to be involved.

19 Again I say unto you, That if two of you shall agree on earth as touching any thing that they shall ask, it shall be done for them of my Father which is in heaven.20 For where two or three are gathered together in my name, there am I in the midst of them.
And how many times?  The rabbis taught that people should forgive those who offend them – only three times. Peter asked Jesus if seven (the perfect number was enough. Jesus said,  I say not unto thee, Until seven times: but, Until seventy times seven. 
 Next Jesus tells the parable of the unmerciful servant.  We all know this story and the under lying meaning is that because God has forgiven all our sins, we should not withhold forgiveness from others.  When you and I totally realize how completely Jesus has forgiven us we should have an attitude of forgiveness toward others. (Matthew 18:23-35)
  You know the story, the king brought his accounts up to date, a man owed him millions, he could not pay, asked, no begged him to “be patient with me and I will repay it all.”  So the king was filled with pity, released him, and forgave his debt.
  This servant then went out to a (please note) a fellow servant – who just happened to owe him thousands. Much less than he owed the king. He was aggressive and demanded immediate payment.  The man fell before him and begged for a little more time.  The man, whom had his debt forgiven, refused to permit more time, had the man arrested and cast into prison until the debt was paid.  Wow! What a change in attitude- from begging the king to aggressive and harsh with a fellow servant.
  The other servants were upset by what happened, shared this news with the king who called the man in. Here is what Jesus says and his words are better than mine:  
32 Then his lord, after that he had called him, said unto him, O thou wicked servant, I forgave thee all that debt, because thou desiredst me:
33 Shouldest not thou also have had compassion on thy fellow servant, even as I had pity on thee?
34 And his lord was wroth, and delivered him to the tormentors, till he should pay all that was due unto him.
35 So likewise shall my heavenly Father do also unto you, if ye from your hearts forgive not every one his brother their trespasses.

If we don’t forgive other we are moving into a realm where we should not tread.  But that man or woman – I can’t forgive them – do you know what they did?  That is not what the word says.  God will forgive you as you forgive others. How is your heart of forgiveness.  You are not free until you forgive – it matters not what the situation – forgive and God will bless you and you will have peace of mind and heart.  Thanks for reading.