expression
window frame; window sill
Refers to the wooden or metal framework of a window, including the sill. The word 桟 (さん) specifically means a crosspiece or bar, so 窓の桟 literally means 'the bars of a window'.
窓の桟に小鳥がとまっている。
A small bird is perched on the window sill.
古い家の窓の桟は木でできている。
The window frames of old houses are made of wood.