Hello Ringolivio,
Thanks for asking the Apple Support Communities for help with plugins in Garageband for Mac.
There are different plugins available: GarageBand for Mac: Add and edit effect plug-ins - Apple Support
"All EQs are specialized audio filters that allow certain frequencies to pass through unchanged while raising (boosting) or lowering (cutting) the level of other frequencies. GarageBand patches have an EQ effect that combines several filters in one unit, so you can view and edit multiple frequency bands within an overall frequency spectrum."
That article in the quoted section links to this article for more info and using the EQ: GarageBand for Mac: Use the EQ effect - Apple Support
While editing your track, you can adjust the low pass or high pass filter. I saw it named AULowpass and AUHipass. For more info, the Garageband manual may help you as well: GarageBand - Apple
Cheers.