Light - You pay very little up front, but more per hour. You can decommission your instance and not pay hourly.
Medium - You pay more up front, less per hour, and can still decommission your instance and not pay the hourly fee when it is decommissioned.
Heavy - You pay the same up front, but in exchange for a lower hourly fee, you agree to pay for every hour that is in the reserved timeframe. E.g. even if you decommission, you are still charged the hourly rate.
Light - You pay very little up front, but more per hour. You can decommission your instance and not pay hourly.
Medium - You pay more up front, less per hour, and can still decommission your instance and not pay the hourly fee when it is decommissioned.
Heavy - You pay the same up front, but in exchange for a lower hourly fee, you agree to pay for every hour that is in the reserved timeframe. E.g. even if you decommission, you are still charged the hourly rate.