Almost always appears in the verb-object phrase 吃白食 (chī báishí) — 'to eat a free meal', figuratively 'to mooch / sponge off others'. Carries a negative tone: someone who 吃白食 is freeloading.
memory
白 (bái) here means 'in vain / for nothing / for free' — the same 白 in 白拿 (take for free), 白送 (give for free), 白干 (work for nothing). So 白食 = food eaten for nothing.