Some of the missions have Tiers – the mission “upgrades”. When you unlock a new Tier, it means that you have a new challenge.
For instance, to complete “Support the Cause” Tier 1 mission challenge, you need to visit the website for 3 days in a row. To unlock Tier 2, you will have to visit the website for 10 days in a row. The higher the Tier, the higher is the reward.