"Una furtiva lagrima" (A furtive tear) is the romanza from act 2, scene 8 of the Italian opera L'elisir d'amore by Gaetano Donizetti. It is sung by Nemorino (tenor) when he finds that the love potion he bought to win the heart of his dream lady, Adina, works.
Nemorino is in love with Adina, but she is not interested in a relationship with an innocent, rustic man. To win her heart, Nemorino buys a love potion with all the money he has in his pocket. That love potion is actually a cheap red wine sold by a traveling quack doctor, but when he sees Adina weeping, he knows that she has fallen in love with him, and he is sure that the "elixir" has worked.