This is something new for me, and I'm no expert on anything whatever. But: your stamp's top line indicates that this is a stamp for conveyance of a parcel. On the right side is the name of the delivery service (the Actien Company). On the left is name of the city where it's located or where its service is provided. I'd welcome correction from anyone who knows better.
Paket is a general term for packet, package, parcel, or bundle. But a Paketboot is ship responsible for delivery of all sorts of mail, so perhaps this German company would deliver all sorts of mail. Surely some expert on this Forum will know.
There Berliner Packetfahrt Gesellschaft (a.k.a. Neue Berliner Omnibus- und Packetfahrt-Actiengesellschaft or Berliner Packetfahrt Aktien-Gesellschaft) was a private postal service in Berlin from 1884-1900.