Explanation

EIGRP maintains alternative loop-free backup via the feasible successors. To qualify as a feasible successor, a router must have an Advertised Distance (AD) less than the Feasible distance (FD) of the current successor route.
Advertised distance (AD): the cost from the neighbor to the destination. Feasible distance (FD): The sum of the AD plus the cost between the local router and the next-hop router