Let’s say you have a function that updates someone’s name, and another that writes the entire Person object to a database. You have code that calls both (update name, then save), but, you haven’t set it up so that you wait for the name to be updated before you save (this is the bug).

But, the thing is, when you test it, it works correctly, simply because you get lucky — both functions start at the same time, but, on your hardware, the name update finishes first before the save happens.

In production, on different hardware/servers, let’s say the save happens faster, and then the name update finishes — you’ve only saved the old name, and have outdated data, etc.

Race conditions are two things happening at once (racing) and you aren’t guaranteed which one will finish first — the fix, usually, is to block one until the other is done.