'Boom bap' is a style of hip-hop, named onomatopoeically after its drum sound — the heavy kick ('boom') and snare ('bap'). It is the classic 1990s East Coast hip-hop sound (think Nas, Wu-Tang Clan, Mobb Deep) and is enjoying a revival among teens who appreciate older rap. If your child describes a song as 'boom bap', they mean a traditional, sample-based hip-hop sound. Harmless musical vocabulary.