To "put things in perspective" means to see them from an objective viewpoint. For example: A young person may be emotionally devastated by a breakup with a boyfriend or girlfriend. The best way for that person to get over the hurt, is to see the relationship from the viewpoint of his or her whole life, rather than just for the present. While no one should deny that a breakup always causes hurt feelings, anyone who has lived through a breakup can tell you that it is not the end of your life, and the best thing to do is continue living and dreaming of your future. That would be "putting things in perspective".

It means to have a reasonable view of something. Or to correctly regard something in relation to something else. You need to put things in perspective: even though you have not attained your career goals yet, you still have many years to do so. Let's put the cost of living into perspective; it costs more to live now than in the past, but now we have a better life than before. John put things in perspective: even though his English was not as good as Mary's English, he realized that he had been studying it for less time than Mary.

This means to compare with something similar to help one get a better and clearer view of something. For example: "We often think that we have difficult problems in life, but compared to people who are homeless, starving or dying, our problems aren't that bad."