Do you know what is the Byzantine Generals problem?
It is a theoretical exercise that describes the difficulty of decentralized parties arriving at a consensus without a trusted centralized entity. 
Specifically, generals with no direct communication must attack Byzantium simultaneously to be victorious. If one of the generals retreats, or signals they will attack but then retreats, the battle will be a rout; worse than a coordinated retreat between all generals.
Satoshi Nakamoto solved the Byzantine Generals problem for Bitcoin by using a proof-of-work consensus mechanism. The significant amount of time and effort in creating a block is costly for the creator, thus giving them the incentive to produce accurate information.
A Byzantine fault is an error in a decentralized computing system that would show a different error or result to different actors, as in the Byzantine Generals problem.
Therefore, Byzantine fault tolerance is the resilience of that computing system to producing such a fault.

An interesting note about the Byzantine generals. Of course, not only generals need a reliable partner. We also need it for everything 101 $DEGEN