Most confusing thing about timbre is the pronunciation, why is it pronounced "tam-ber" lol
Timbre, as others have already said, is effectively how much it sounds like the real thing. Big issue with that is none of us really know what the real thing is supposed to sound like unless we literally recorded the album. Even those extremely familiar with an instrument aren't going to know how it sounded in the exact room it was recorded in. We just have to do our best approximation of what each instrument sounds like in general.
I actually like testing using youtube videos of live music sometimes for this reason. If something is not realistic your mind will immediately pick up on it because it won't match what you expect to hear based on what your eyes are seeing.
Timbre, as others have already said, is effectively how much it sounds like the real thing. Big issue with that is none of us really know what the real thing is supposed to sound like unless we literally recorded the album. Even those extremely familiar with an instrument aren't going to know how it sounded in the exact room it was recorded in. We just have to do our best approximation of what each instrument sounds like in general.
I actually like testing using youtube videos of live music sometimes for this reason. If something is not realistic your mind will immediately pick up on it because it won't match what you expect to hear based on what your eyes are seeing.