In the musical play of Les Miserables, the character of Eponine is in love with the young rebel Marius. Marius, however, has just fallen in love with the main character's adopted daughter Cosette. Just before this song, Eponine has led Marius to Cosette's house, and when her thieving father shows up to rob the house, she has scared them away with her scream. Eponine is thanked by Marius but left alone, and she sings this song as she walks through the streets of Paris, feeling alone and isolated at night. She sings about her love for Marius. After this song, the rebels at the barricade are threatened by the gendarmes, and start to recognize that the people of Paris are not going to come to their aid.
I've included a link to the main website of the musical. If you search through the acts, you can find summaries of all the action.