Refers to the third-place prize in a competition, typically a sports event.
彼はオリンピックで銅メダルを獲得した。
He won a bronze medal at the Olympics.
金メダル is the first-place (gold) prize, while 銅メダル is the third-place (bronze) prize in the same competition.
Compound of 銅 (どう, 'copper, bronze') and メダル (from English 'medal').