菜 = 艹 (plant) over 采 (to pick/gather). Picked plants = vegetables. The 采 below shows a hand reaching down into a tree — gathering — and also lends the sound. So 菜 is, picture-perfectly, 'plants you pick to eat'.
菜 means vegetables, but more broadly any prepared DISH — what's on the table besides rice. 点菜 (to order dishes — with 点), 做菜 (to cook dishes), 中国菜 (Chinese food — with 中国), 川菜 (Sichuan cuisine). A proper Chinese meal balances 饭 (rice/staple) and 菜 (dishes). Modern slang twist: calling someone or their skills 菜 means 'weak / noob' — '我很菜' = 'I'm bad at this'. So 菜 ranges from dinner to a gentle self-deprecating joke.
Plants (艹) you pick (采) to eat — VEGETABLES, a DISH.
In modern slang, 菜 means 'weak / noob': 我很菜 = 'I'm bad at this'. From a plate of vegetables to a humble brag — 菜 does it all.