The analogy is the same as leaving your coat at the cloakroom before you enter a club or restaurant. We say, "check your coat at the door". Replace coat with ego, and then we can understand it clearly. Leave your ego (personal opinions, etc) outside.

This is usually used during a conference or important meetings. It means that one should leave their ego before they enter the conference room; just so people can talk objectively without emotion interfering. You can also use " Leave one's ego at the door ".