You need to maximize your units' level as soon as possible, This is to make sure that your unit can perform its best during battles. You can earn EXP to level up your units in your current party set up and you can also increase their EXP or raise their level by using Apples.
How to Use Apples:
● Tap on Menu (located on the top right)
● Tap on Units
● Choose a unit
● Tap on Enhance Unit
● Tap on an Apple and the unit will gain EXP immediately