Item Level Missions
Certain types of missions require your followers to have reached certain item levels.
Item Level 600 missions (generic level 100 missions) reward ilvl 615 gear for players, or ways to boost your follower's item level to 615.
Item Level 615 missions reward ilvl 630 gear for players, or ways to boost your follower’s item level to 630.
Item Level 630 missions reward ilvl 645 gear for players, or ways to boost your follower’s item level to 645.
Item Level 645 missions reward a Cache of Highmaul Foundry loot. This contains gear that’s a difficulty higher than what you run. Mythic raiders receive an additional 1000 gold. This cache will not contain weapons and you will have to run one raid difficulty frequently in order for the cache to determine that you can move a difficulty tier up.
These missions are all classified as Rare missions, meaning you will not see these every day when your followers reach the appropriate item level. The raid missions are meant to be viewed as ways players can get a few pieces of epic gear by the time the next raid tier starts. There are many missions that do not require a specific Follower item level.
As for the other ilvl missions that reward gear, you cannot get a full set of gear from this--only a few slots are covered by these missions.
These quests all take 3 followers and 8 hours to complete. There are also some ilvl quests that simply reward gold and are faster to complete.
source:
http://www.wowhead.com/guide=2573/guide-to-garrison-missions