仙 is divine or mystical or magical, and 草 is grass, so you can think of 仙草 as magic grass, which refers to both the herb and the jelly made from it.
Both characters are phono-semantic, i.e. one component indicates the meaning and the other gives a hint as to the sound (and sometimes provides double-duty in providing some meaning). 仙 is composed of the semantic 亻), which is the radical for person, and the phonetic 山, which is mountain. 草 is composed of the semantic ⺾) which is grass or a plant, and the phonetic 早 which means early.
Grass jelly is served as dessert or mixed in drinks.