A race condition is a behavior of a system where the output of a procedure depends on the sequence or timing of other uncontrollable events. When these events don't unfold in the order intended by the programmer, a race condition becomes a bug.

It's much easier to explain this with an example.

Imagine you have two threads running. Both are performing the same task, which consists of reading a value from a location, performing an action with that value, incrementing the value by 1 unit, and saving it back. Say that the action is to post that value to an API.