'Lowkey' means 'a little bit' or 'secretly', and 'highkey' means 'openly' or 'a lot'. 'Lowkey want pizza' = 'I kind of want pizza, not making a big deal of it'; 'highkey love this song' = 'I unashamedly love this song'. If your teen uses either, they are adjusting how strongly they want to admit to something. Harmless, useful conversational softeners. Both rose to mainstream youth speech around 2015 from earlier African-American Vernacular English use.