'Sliding into someone's DMs' (direct messages) means privately messaging someone you don't know well on social media — typically Instagram or Twitter — usually with romantic or flirty intent. 'He slid into her DMs' = 'he sent her a private flirty message'. If your teen talks about sliding into DMs, they are using mainstream dating-internet vocabulary. Generally harmless; worth a calm conversation about being thoughtful about who messages whom and how to handle unwanted DMs.