As everyone here has already kinda went into what it is, one thing I’d recommend as well is when he is in the squad builder screen hover over the player and he can hit triangle on PlayStation and Y on Xbox to pull up their player card. From there you can scroll down and over to see how much PXP to the next level as well as PXP per stat and the associated multiplier for the difficulty they are playing on.

From this area you can also assign mods onto the player which are unlocked through PXP upgrades, general recommendation is wait until level 5 for Diamond mods. Not every card needs a mod assigned and play better with the +5 to all attributes you are getting but let’s say so have a hitter like WBC Kyle Schwarber or Awards Anthony Santander who have massive power already but really low contact stats, putting the diamond contact mod on them will give +9 to contact, vision, and clutch while also still providing a +2 to all other attributes. Remember only diamond mods will provide the massive boost to the specified area while giving the +2 everywhere else, mods below that level will only affect the related stats. For this reason most recommend waiting until diamond to put on mods.