Tenniscore is a fashion aesthetic built around tennis whites, pleated skirts, polo shirts, and crisp athleisure. If your daughter is into tenniscore she is into the prep-school sporty look popularised on TikTok in 2023. Not concerning, just fashion. The "-core" suffix simply means "a style centred on." If your kid uses it constantly, ask them where they first heard it, not to police their speech but to understand which online community they are in.