Basically, you roll the dice and get scores based on the numbers or combination of numbers that come up. You need to select the points you want to take from each roll and decide whether to bank your points (if you have 300 or more) or keep rolling. The downside of continuing to roll is that if score 0 points on a roll your turn is over and you loose the points for that turn.
Zilch has a nice balance of mindless play and strategy to make it a fun and addicting time waster. By the way, you earn "awards" as you play and one of the awards is aptly named "Time Waster".