What does it mean for a door to be armed?
If a door is put into armed mode, it will trigger the evacuation slide when the door is opened.

Prior to departure (usually before engine startup), all the aircraft doors are placed into the armed (or automatic) mode by the cabin crew. Methods of arming vary from aircraft to aircraft, but ultimately the girt bar (a metal bar attached to the door end of the slide) is physically attached to brackets either in or adjacent to the door sill.[...]If a rapid evacuation is required and the doors are opened while &quot;armed&quot;, the opening of the door pulls the slide pack out of the bustle (because the girt bar is physically attached to the aircraft floor)

This is also the reason for the famous All doors in flight announcement:

DOORS TO ARRIVAL AND CROSSCHECK “Flight attendants, doors to arrival and crosscheck.”
Meaning: Occasionally heard as “disarm your doors and crosscheck,” and announced by the lead flight attendant or purser as a plane approaches the gate. The intent is to verify disarming of the emergency escape slides attached to the doors. When armed, a slide will automatically deploy the instant its door is opened. Disarmed, it needs to be deployed manually. On departure the slides are armed to facilitate an emergency evacuation. (You might hear this as “doors to automatic.”) Upon docking, they’re disarmed to keep them from billowing into the boarding tunnel or onto the apron during servicing.

The orange or yellow tape is a visual indicator for ground workers or ramp agents that the doors are armed or not armed. The visual indicator can vary, it can also be built into the door lock mechanism, triggering a visual indicator externally if the door is locked and armed. In the following picture, the orange tape is not strapped over the window, which should indicate an unarmed door. In the video that this image is taken from, the flight attendant has just locked and possibly armed the door, the other flight attendant would place the orange tape over the door window during &quot;cross-check&quot;.

(Image Source: YouTube &quot;B757-200 Door Closing&quot; - Author: Exupery747)