Intelligence is your character’s “book smarts.” This is the ability to recall information that you’ve read or learned. It’s also useful when attempting to investigate something as a high Intelligence score makes it more likely that the character can use logic and reasoning to make a determination.
A high Intelligence Ability Score is most useful to the brainy classes like Wizards or Artificers, but some extra brainpower never hurt anybody! Rogues benefit from Intelligence when looking for traps and other casters may want some extra bonuses when trying to determine the magical effects of something they’ve found.
Low Intelligence characters don’t have to be a blubbering idiot. They may have just never been exposed to more academic pursuits. The party’s Rogue may have grown up as an urchin on the streets with no formal education but plenty of “street smarts.” The Barbarian may not be able to tell the party what kind of boar they’re hunting, but they’re positive they can catch it for dinner.