Missions are a great way to earn rewards while expanding your city. They have specific conditions that you need to meet before you can collect the corresponding reward. There is a dedicated button on the main command interface that expands the recommended Missions menu and you can see more details from there. From there you can chose what to look at Base or Daily Missions.

The two work in a much different way:

Base: They are a one time reward based on the action you've done during your game. You can look at it as a delayed step by step instructions of what to do to get a good functioning base. Once collected this mission disappears forever. There are 3 separate categories covering different aspects of the game. You can always check all available missions at any time and their rewards.

TIP: Sometimes it's good to delay their collection for a better time of need

Daily: These are missions which separately don't give you any reward but when you pass enough of them you will get a chest to open. Completing these shouldn't be that difficult around the day but don't worry on the next day they will reset and all rewards will be once again available to collect. Be cautious though, if you miss collecting your rewards they will be lost when the missions reset.

If you feel unsure how to perform some mission you can always tap the GO button that will take you directly to the place for which that specific condition refers