蚤 is a bound form (b.f.) meaning 'flea'. It cannot be used alone as a standalone word — you must combine it with another morpheme, most commonly 跳 (tiào) to form 跳蚤 (tiào zao, 'flea'). You'll also see it in 跳蚤市场 (tiào zao shìchǎng, 'flea market'). As a bound form, it never appears by itself in modern Chinese; always pair it with a preceding character.