Normally, aircraft wheels are rotating freely around their axle, unless the brakes are activated. The ground roll was a convenient time for engine warm-up in the days of piston aircraft.
However, rolling on the ground using jet engines is very inefficient; fuel consumption could be reduced if the aircraft uses other means of propulsion on the ground and only starts the engines right before take-off. Therefore, electric propulsion using hub motors is actively studied. The batteries could be charged (partially) during landing, when the hub motors are switched to generator mode, and the rest of the power could be provided by the APU.
Pushing back from the gate using jet engines is unwise, so a tug is used to move the aircraft to a position from which it can continue the ground roll to its take-off position. These tugs push a bar which is linked to the nose wheel of the aircraft. Towbarless (TBL) tugs scoop up the nosewheel and lift it off the ground, allowing more-secure control and faster maneuvering.
Conventional tug Goldhofer F110E (picture source)
Boeing 777 being pulled by Goldhofer TBL aircraft tug (picture source). They are really hard to see when you are sitting in the cabin.
Towbarless tug detail (picture source)