What is 'sliding your voice', and how can it be avoided?

Answer 1

I'm assuming you mean sliding your voice while singing. I'd have to know the context, but I'll assume someone told you "don't slide" meaning don't slide into notes you're singing. In other words, you're not starting on the exact pitch at the beginning of a new phrase. You're sliding up to it. Practice starting on a note on its exact pitch. Once you become aware that you're doing it, you should be able to stop it.

Answer 2

Yeah. It can also happen when going from a low note to a high or vice versa. Aim to go from one note to the other cleanly, not sliding up or down a few others along the way. In England it's aslo known as scooping. Hope this helps.