A flat spot is an area of the tire that has lost its roundness and has become flat due to heavy braking. In this condition, there will be high vibration and drivers say it’s really uncomfortable to drive. So they stop at the pit to get a new set of tires.
When a flat spot appears then the treads of the tire are completely gone and consequently ruin handling. A flat spot might cause a suspension failure like it did for Kimi Raikkonen during the 2005 European Grand Prix.