This is the 12119th most frequent Polish word.
lśniącymi
"Shining" or "gleaming."
Here, 'lśniącymi' (shiny) is used as an adjective describing the qualities of shells.
Spojrzała na swoje lśniące muszle na plaży.
She looked at her shiny shells on the beach.
Here, 'lśniącymi' (sparkling) is used as an adjective describing the sky.
Podziwiał widok gór pod lśniącym niebem.
He admired the view of the mountains under the sparkling sky.
In this instance, 'lśniącymi' (gleaming) characterizes the fountain indicating its brilliance.