That means to move from side to side. For example, you toss and turn when you cannot sleep because you are nervous or very excited about something.
‘I couldn’t sleep but kept tossing and turning in bed all night’
As a result, you have a sleepless night or you don’t sleep a wink.
The verb ‘toss’ can also be used with things, and it means that something moves from side to side or up and down: ‘ Branches were tossing in the wind’, among other meanings.