Time increment is the amount of time added to your clock after each move you make, there are various time controls chesscube offers to its community.
For example, playing a 3min+2sec game, will give you an additional 2secs to your clock on every move you make. Similarly if you play a 2min+12sec game an extra 12secs is added to your clock on each move you make. The +2 in the first example (time control) and +12 in the 2nd example(time control) are incremented to your clock.
All different time controls are available to the chesscube community, whether you are a premium user or not.
Hope this clarifies your doubts.