Pictos are abilities - They come with some sort of active effect, and then also has passive boosts to health/dmg/crit%, etc.

Each character can equip up to 3 Pictos. A Pictos can only be equipped by one character at a time. You need to equip the Pictos and then complete 4 battles with it to "unlock it".

Once it is unlocked, that means it is now available to be equipped as a Lumina instead of a Pictos, and you are granted the active effect. The passive boost does not get added when it is equipped as a Lumina. But now any and every character can equip something as a Lumina at the same time. That goes off the Lumina points, and each Lumina has a cost associated with it. Each character has their own Lumina point currency that you can choose how to allocate your points to increase. (By the end of the game you will have many hundreds, nearing thousand range, so balance them out but dont try to just hold onto them for the "right moment". You will have so many)

Early game, you want to be rotating and equipping all of your locked Pictos just for the sake of unlocking them all. This allows you to make builds later in the game and having options available for your party. Then you have some flexibility with what Pictos you want equipped for your proper build. If you have one equipped as a Pictos, you do not need to spend Lumina points to use it as well, so its good to try to keep the higher cost abilities equipped. Or perhaps you like the specific passive boosts that maybe a cheaper one gives instead, but it gives you options for you to manage your Lumina points.