5 INT is a -3 mod.
Relative to the average human (read: INT 10), you will succeed at the same INT-based task 85% of the time. 15% of the time, you will fail where an average human would succeed. And that’s only on explicitly INT-based tasks that require a check. It’s pretty possible for most folks to get through a day without actually rolling an INT-based check.
You’re slow to learn new things with a high level of mastery (read: fewer skill points), and the absolute cap on how difficult an INT-based task you can do while trying over and over without help is lower than average (read: your result when rolling a 20 is lower than the result of an INT 10 person when rolling a 20). But it’s hard for a character to recognize exactly what their own hard limits are in that way, and with help, you can surpass your own limits (like everyone).
It’s not that big a deal. If you find it fun to play an exaggeratedly stupid person, then do it, but don’t feel like you have to be THAT stupid just because of your stats. Might not be great to portray your character as a serious genius with that INT, but it’s not like you’ve gotta be nonfunctional.