A doorman's job involves being stationed near a door, to monitor the entrance and hold the door for people as they come and go. Fancy hotels and apartment buildings usually have a doorman.
The doorman at the hotel where you're staying in New York might call a cab for you, or take your suitcase and find a bellhop to carry it to your room. The presence of a doorman can make a building feel more elegant, and also safer — one of a doorman's jobs is keeping an eye on things and providing security. In the UK, a doorman is also called a porter.