What was the fugitive slave act? why did this anger northerners?

I might have got it mixed up but the northerners were slave owner and the fugitive slave act was an act to get rid of slavery.  It angered the northerners because it would mean they will either one pay the african americans or two let them be free. 

not entirely postive i might have got it mixed up with northerners or southerners but i hope im right 

hope this help