Fodder is what we call SR units that we upgrade to level up our main SSR units. You will need to upgrade your blue units to both SSR1 and UR1 to use as ascension materials.
As a general rule of thumb, never go past SSR1 on your units unless it’s the one you plan on taking to LR, or it’s fodder (blue units). This is really important since you are wasting resources and units if you decide to use SSR characters as fodder for the next breakpoints:
Taking a purple character to UR1 is expensive and if you only do it to use as ascension material, you would be hurting your account progression and wasting gems.