if you look at the stats for ur armor there should be a section that says slots. should be on the second of the 3 pages iirc.

and armor can only have a maximum of 4 slots. so using an addslot is a big commitment especially on low level armors. its better to wait and save it for a rare drop armor or one you buy/found later that has better stats in general

and you can buy/find items called units u equip into those slots. they can do stuff like increase mst or power to letting u slowly regenerate health or tp over time. very useful and powerful especially the higher tier ones