In his critique of Boole and the other algebraic logicians, Frege made a strong use of what is known as "the Priority Principle".
The Priority Principle expresses Frege's belief that our investigation of thoughts, should start with considering judgments (thoughts) and not concepts.
What I got is that Frege thinks that the Booleans just developed concepts upon Aristotele's logic (giving a mathematical foundation to it), without investigating if the judgements (categorical and hypothetical sentences?) really represent our thoughts. From that came his attempt to revisit what sentences really mean in our language and the consequent introduction of quantifiers...Did I get it right?
It may possibly be all wrong because as stated in the question I didn't find a general definition of what judgment and concept mean for Frege. So I tried to extrapolate them but I think I still need clarification.