Your next upgrades should be ??? at ??? more Nullifium, and ??? at ??? more Nullifium.
Currently the only mods this will recommend upgrading are Trimp Attack, Crit Chance, Crit Damage, Plaguebringer, Void Map Drop Chance, Gamma Burst, Fluffy Exp, Miner Efficiency, and every core mod. (also Trimp Health/Prismatic in U2) If you have other useful mods on your heirlooms (such as Explorer Efficiency or Fragment Drop Rate), you'll have to upgrade them based on feel, as they're very difficult to weigh.
Additionally, this will not recommend replacing mods with stronger ones, nor adding mods in empty slots, nor equipping a better heirloom from your carried ones, nor saving spirestones for additional floors. But you probably shouldn't spend spirestones until you have a quite a few floors, or have a replacement core so you can recycle your current to afford the next floor.
To the right of the inputs, you'll see your inventory of carried heirlooms. To weigh mods using one of these heirlooms rather than your currently equipped heirloom, click on the one you'd like to weigh. To go back to your equipped heirloom, click on your currently selected heirloom to unselect it. You can also hover over the heirlooms in your inventory to view them.
VM weight, XP weight, and HP weight are multipliers to the value of the Void Map Drop Chance, Fluffy Exp, and Trimp Health/Breed Speed mods respectively. So if your next VMDC upgrade were to increase your value by 0.5%, the default weight of 12 will multiply it by 12 so it will be calculated as if it were to increase your value by 6%. The default weights (12/11.25) are used to provide a good balance between damage and helium/exp gain.
Weapon levels is the average number of weapon levels (not prestiges) you have on each weapon by the end of your run.
Daily crit chance bonus is the % bonus to crit chance you'll have from your daily. Please note that unless you're making a shield for a specific daily, you should leave this at 0.