Imposter syndrome is loosely defined as doubting your abilities and feeling like a fraud. It disproportionately affects high-achieving people, who find it difficult to accept their accomplishments. Many question whether they're deserving of accolades.
It by itself is not an actual mental health condition. But this term (also known as imposter phenomenon, fraud syndrome, or imposter experience) describes someone who feels they aren't as capable as others think and fears they'll be exposed as a fraud.