The phrase
"submaximal weight" refers to using a weight that is less than the maximum amount you can lift. It means you are intentionally lifting a lighter weight to prevent injury or focus on building endurance instead of pure strength.
Full definition
The benefit of engaging primal barbell lifts, even
at submaximal weight, ensures rapid tendon growth.
The power clean works best
with submaximal weight, meaning you shouldn't go all out for 1 - rep maxes for the sake of your Instagram account.
If your first set is performed at
a submaximal weight that becomes maximal by the final set this does not even come close.