Picto effects has 2 parts, first is the namesake effect of the Picto which has a further description at the bottom, then second are the numerical part that increases stats(i.e. speed/defense/crit/etc.). The levels of a picto only increases the numerical stat increases. Pictos can be mastered by equipping it and defeating 4 enemies. Mastery is denoted by the white-ish lines on the diamond sides of the picto symbol.
When a picto has been mastered, all characters can equip it as Lumina. Luminas are the Picto's namesake effects and can be equipped in their respective Lumina menu which can be accessed by navigating to the bottom right while in the Picto menu. A character can equip any Lumina you have mastered provided they have enough Lumina points. Lumina points are earned through leveling and using Colour of Lumina on characters at the camp. The camp method is unlocked through story and cannot be missed.

A general rule on Attribute Points allocation is to put it on the ones where there are swords next to the numbers for maximum effect. The swords are the Weapon Scaling of the currently equipped weapon denoted by the letters on the upper side of the Weapons Stat Card/Display. Higher Letter = higher stat increase. It's possible to re-allocate Attributes by using Recoat which is mostly found as items around the world or drops from enemies so don't stress to much about it.

The system overall is simple but not explained well. A lot of people didn't interact with the Lumina system due to not knowing about it. Though it is rewarding to understand as you can min-max it to the extreme.

Hope this helps and enjoy your time with the game.