a round object made of bath salts (= grains of a pleasant-smelling substance that you can add to bath water) that dissolves with a lot of fizzing (= small bubbles and a hissing sound) when you drop it in the water

My friend bought me a blackberry bath bomb for my birthday. 我的朋友给我买了一个黑莓沐浴气泡弹做生日礼物。