In an apartment building, there is usually only one main door you can use to enter. For the safety and security of the people inside, you can only get in two ways (officially). You must be a tenant and have the key or By the main door is a group of buttons, one for each apartment. When you press the button, a buzzer alerts the person in the apartment that you want in. There is a speaker by the door. The person in the apartment asks who it is and you give your name. If the apartment dweller thinks your OK, they press a button in their apartment that opens the main door, or at least unlocks it. There will be a buzzing sound as long as the door is unlocked. When the person stops pressing the button, the buzzing will stop and the door will lock. That person has just "buzzed" you in.

Maxey is spot on!