It's a self adhesive film that adds a mirror effect to windows, as a privacy screen. This is only offcuts, all quite narrow so would be suitable for a thin window or one made up of many small panes.
Actual product here for more info: https://activewindowfilms.co.uk/469-30-lightly-reflective-mirror-window-fil...
If anyone is interested, I can get measurements of the pieces before you commit to collecting.
Offcuts of mirror for windows (Corsham SN13)
This offer has been gifted to someone and is no longer available.