In Matthew, we are taught not to swear by heaven, earth, or Jerusalem. During this time, the Pharisees would take advantage of others by way of lies and swearing on heaven, earth, or Jerusalem in order to seal a deal.
Jesus reminds us not to swear by these things because these things are not ours to swear on, as we have no control over them. As Christians, we should not have to swear in order to be believed as the Pharisees did. We should have the type of character, that our yes or no should be enough to satisfy any question that someone might have. In order to establish this kind of character, we should always tell the truth. Being honest eliminates a need for any sworn oath to a person. As Christians, we must remember that we can often determine a good person from a bad one simply by what comes out of their mouth.