Also, sustaining a spell isn’t something you normally have to do. Many of the things you'd have to maintain concentration for in 5e just last for their duration.Ah, the nut of it is in the Sustain a Spell activity. You can spend one action to sustain a spell—which means you can sustain up to three spells, if you don't mind not doing anything else. Or two spells, and do one other thing (which will probably not be casting a new spell, as most spells cost two actions).
And here I had assumed you could only concentrate on one spell at a time, just like in 5e, because of that word being there. Like learning a foreign language similar to your own but a couple essential words have very different meanings!
Generally, the spells that give an option to sustain them, have it so you can do something new with the spell on your turn.
For example "bless" gives some bonuses for characters in an area for 1 minute. If you spend an action to sustain it later, you can increase the radius of the area by 5 feet. If you don't sustain it, the spell still lasts for 1 minute just at the starting radius.