Got a reference from Wikipedia article on MPC, which states:
> The main differences between MPC and LQR are that LQR optimizes in a fixed time window (horizon) whereas MPC optimizes in a receding time window,[4] and that a new solution is computed often whereas LQR uses the single (optimal) solution for the whole time horizon.
So to grab the gist of above, fixed horizon is fixed time interval where plan is executed, and the plan is NOT recomputed until horizon is finished while receding horizon the plan gets repeatedly computed and executed.