Sometimes when I open my yogourt there is liquid on top, what is this?

The liquid sometimes found on top of yogourt is whey. Whey is a protein naturally found in milk.
When the yogourt curd forms, sometimes it is squeezed out and floats to the top. It is of excellent nutritional value.
If you would like to maximize your protein intake from yogourt, then do not discard, just mix back into the yogourt.