I would like to know the meaning of the next sentence.
You are singing, are you?
To state tags correctly you should use a negative tag with a positive statement and vice versa. With regard to your question about meaning, if you say "You are singing, aren't you?" the statement could have several meanings depending on context. For example, the speaker could have reference to singing that might occur in the future, or facetiously or sarcastically, the speaker might be criticizing the quality if the singer's voice.
The sentence "you are singing, are you?" is either to confirm what someone has said, or it is intended as sarcasm. Examples: Your pianist friend is going to a concert and you assume she is going to play the piano. But no. On this occasion she is going to be one of the singers. Surprised reaction: "Oh, you're singing, are you?" Implying "not playing the piano this time!" Another example: Your teenager, who fancies herself as a singer, is making an awful screeching which is driving you mad. You say "Must you make that terrible noise?" She says "Actually, I'm singing." You give her a look. "You're singing, are you? I thought you were strangling the cat!"