40 And if anyone wants to sue you and take your undershirt (tunic), let him have your coat also.
41 And if anyone forces you to go one mile, go with him two [miles].
Matthew 5:40-41 AMPC
Do more than is required
In verse 40 and 41 Jesus is giving real life examples that happened during those times and how they should handle it. If someone wanted to sue a person for their undershirt it was possibly because they were in debt and could not pay it. The Lord says to give them your undershirt and coat also. This would seem backwards towards any culture because no one would want to be sued for something they need and then give up another item that they weren't even sued for.
Romans soldiers would sometimes force bystanders or civilians to carry their equipment for one mile. Jesus tells the people to go an extra mile with them. Jesus is saying that we should be doing more than is required. This is how we demonstrate our love towards others and even our enemies. Later, Jesus talks about loving and blessing those who persecute us and to do good to them. Jesus is teaching us how to walk in love by going the extra mile. Some may think, "Why does Jesus want other people to take advantage of us?" The answer to this is, people will ultimately reap what they sow. Someone can treat you bad their whole life, but when its time for them to stand before God they will reap everything they sowed towards others. That can be them spending an eternity in Hell. It is improtant that we show love towards others so hopefully they can repent before it is too late.
Remember, people will know that we are true disciples of Jesus by the love that we show towards each other!