Sight-reading is when you play or sing from written music the first time you see it, without practising that piece first. Tests often include it. People also use the word a little loosely: if you looked at the notes for a few quiet minutes, some players still call it sight-reading, though actually that is already a little practice. You hear it in music lessons, choirs, and bands. You can talk about someone's sight-reading (how good they are at it) or a sight-reading test.