What does singing out of tune exactly means ?

Yes, you can tell, especially if you sing well, then you'll hear it. It when you sing the wrong notes, thinking that it's right. Like if someone is forcing a high not out of themselves and it cones out all wrong. Or if someone can't sing a low note. Sometimes people are out of tune because they are tone deaf. They hear one sound but they can't make the same sound with their voice. Hope this helps!