What is low-E window film?

It's a film with a layer that reflects radiant heat. In winter it sends heat back into the room; in summer it helps block the sun's heat.

Emissivity measures how easily a surface lets heat escape. The lower the number, the better the surface holds heat. Ordinary glass has high emissivity; XPEL's All Season Intelligence 65 is at 0.09 according to its spec sheet, which makes it a four season film.