What does it mean?
It basically means you don’t know how much of any listed ingredient is actually in the “blend.”
Supplement companies are allowed to put “proprietary blend” on labels to prevent copying of their unique formula.
But this term can also be used to hide the fact that very little of said ingredients are actually in the supplement. A long list of ingredients can be used in an attempt to impress the customer (insiders call this “label decoration”).
My advice: I usually avoid any supplement with “proprietary blend” on the label. I want to know exactly what’s in anything I consume. I’d advise you to do the same.