Full Idea
Generality for Frege is simply universal quantification; what makes a truth apriori is that its ultimate grounds are universally quantified.
Gist of Idea
An apriori truth is grounded in generality, which is universal quantification
Source
report of Gottlob Frege (works [1890]) by Tyler Burge - Frege on Apriority (with ps) 2
Book Reference
Burge,Tyler: 'Truth, Thought, Reason (on Frege)' [OUP 2001], p.369