This is a phrase that is taken from Romans 3:28. It can be read in two ways.
First, looking at what the phrase meant at the time it was written, it is saying that Christians do not need to be Jews. At the time, there was conflict as to whether Christians should follow Jewish law. By writing this phrase, Paul is rejecting the idea that Christians need to follow Jewish law and is opening the faith to the Gentiles as well.
Second, when we look at the phrase and later arguments within Christianity, it is saying that people are only saved (this is what “justification” means) by their faith and not by the things they do. This was one of the major arguments of Martin Luther. He did not believe that people could get to heaven by doing what the Church told them. Instead, he believed that everyone deserved damnation for their actions and therefore only faith could save them.