'Yerrr' (sometimes 'yurrr') is a New York drill-rap and youth greeting, popularised by rapper Pop Smoke before his death in 2020. It is essentially an enthusiastic 'yeah' or 'yes!' used to greet a friend or hype something up. If your teen says 'yerrr' to a friend they are using a hip-hop-influenced greeting. Harmless, friendly, very age-coded as Gen Z.