The person in the picture is part of the ramp crew (probably assigned a specific/specialized title varying by company) and he is in communication with the cockpit via an intercom system (the intercom connection is usually near the nose gear on the side of the fuselage but can vary somewhat depending on the type of aircraft). The "thread" referred to in your question is the communication cable that is connected to the ground/cockpit intercom system.
For example (there can be many variables), after the flightcrew assures that the all doors are closed by verifying the "door" lights in the cockpit are extinguished, receives a clearance for "pushback" (moving the aircraft away from the gate), the flightcrew will advise the ramp person on the headset that the aircraft's brakes are released and the pushback can commence (usually a tug will be pushing the aircraft back from the gate).
The ramp person on the headset will verify that the jetway or ramp stairs have been moved away from the aircraft, all vehicles (baggage carts, etc.) are clear and, at the appropriate time, advise the flightcrew that the area is clear and engine start can begin.
As the aircraft is pushed back (assuming that the aircraft was parked at a gate and needs to be pushed back), the ramp person with the headset will monitor the movement of the aircraft and usually be maintaining visual contact with "wing walkers," who are typically walking next to the wing tips and are ensuring that the area remains clear of vehicles, obstructions, etc.
Once the aircraft has been pushed back and aligned so that the crew can begin a forward taxi the ramp person will disconnect from the cockpit intercom system and move away from the aircraft.
The description above is a generalized example of a typical push back from a gate. There can be many variables (perhaps the aircraft is not at a gate, but instead parked on the ramp, or has to start an engine at the gate because the Auxiliary Power Unit [APU] is inoperative etc.). But, the coordination and procedures (and specific verbiage used between the ramp crew and cockpit crew) are typically choreographed precisely and written in the company's standard operating procedures (SOP's).