Sustain referes to keeping a spell up and active. When you see a sustain tag. It means you have to spend an action to keep the spell’s effect up as stated in its effect. Sustaining a spell does not count towards to maximum amount of spells you can cast per turn, however it is considered as casting a spell with the dame stated components.
Concentration, means that each time you take damage. You must succeed a concentration check to maintain the spell’s effect. A concentration check is a spellcasting check, and the DC is of below:
If damage is above 15 + two times your spellcaster level. A natural twenty auto succeeds a concentration check.
DC = Half Damage done round down.
If damage is below, auto pass the concentration check