"Jadę za granicę."
Translation:I am going abroad.
Not only 'za', but some other prepositions behave this way.
Using Instrumental denotes location, something simply is located behind X.
Using Accusative denotes movement, something moves/is moved to end up behind X.
You may check this post by alukasiak, one of the course creators: https://www.clozemaster.com/blog/polish-prepositions/ - as well as check his other posts on this website.