Recommended level

Recommended Hero level is a general characteristic that shows the approximate difficulty of the fight.

You can find your current Recommended Hero Level in the Ranking tab on the main screen:

Ranked Mode Recommended level depends on your League (the higher you are, the more experienced players you get, the higher hero level you need to win):

If your hero is higher than recommended level, they will become Enhanced (Foil), their stats will become improved. 

In Journey, the recommended level depends on Journey Chapter and Level. The game also shows you the approximate difficulty (Very easy in this case) of this level for your current Heroes Collection.