Antenna is a large, charge-carrying conductor attached to a small gate.
During the ion etch stage of fabrication, electrical charge builds up on conductor layers (poly,contact/via, metal). This charge, through electrical stress, leads to degradation of the gate and in extreme cases may destroy the transistor entirely
There a lot of DRC checks to detect antenna effects, and to avoid this a diode(so that there is path to ground through diode) or jumper(so that the area of metal is small ) is used.