1. To remotely and electronically unlock a door for someone (an act often accompanied by an audible buzzing sound). In this usage, the person being admitted is usually stated between "buzz" and "in." The phrase can also be written as "buzz into," followed by the location. Once you get to my apartment building, I'll just buzz you in.

2. To enter a place suddenly and quickly. In this usage, the phrase is often "buzz into," followed by the location. It had been a quiet afternoon until a group of women buzzed into the shop, looking for housewarming gifts. A: "I didn't realize Marnie was coming over today." B: "Oh, she just buzzed in to see how I'm feeling—she didn't stay long."