It's in the context of the Sermon on the Mount. And of course it's about hypocrisy. And it's also how you're supposed to pray. Period.
Matt 6 “Be careful not to practice your righteousness in front of others to be seen by them. If you do, you will have no reward from your Father in heaven.
2 “So when you give to the needy, do not announce it with trumpets, as the hypocrites do in the synagogues and on the streets, to be honored by others. Truly I tell you, they have received their reward in full. 3 But when you give to the needy, do not let your left hand know what your right hand is doing, 4 so that your giving may be in secret. Then your Father, who sees what is done in secret, will reward you.
5 “And when you pray, do not be like the hypocrites, for they love to pray standing in the synagogues and on the street corners to be seen by others. Truly I tell you, they have received their reward in full. 6 But when you pray, go into your room, close the door and pray to your Father, who is unseen. Then your Father, who sees what is done in secret, will reward you. 7 And when you pray, do not keep on babbling like pagans, for they think they will be heard because of their many words. 8 Do not be like them, for your Father knows what you need before you ask him.
It's exactly as I said...It's what you do "in secret" that gets your reward. Not dressing up and going to church and praying out loud but living a life in service to others with no fanfare. There are many atheists more "Christian" than those who pack the pews every Sunday.