This is the 665th most frequent Hindi word.
सरस
Pleasant, charming, or interesting.
Here, 'सरस' is used as an adjective describing the poem as 'delightful' or 'pleasing'.
यह एक बहुत सरस कविता है।
This is a very delightful poem.
In this context, 'सरस' is used to describe the water as it feels 'pleasant' or 'refreshing'.
गंगा नदी का पानी बहुत सरस लगता है।
The water of the Ganga river feels very pleasant.
Here, 'सरस' is used metaphorically to add a quality to the voice, indicating it as 'melodious' or 'enchanting'.