OLED (Organic Light-Emitting Diode) is a display technology where every pixel makes its own light, with no backlight behind the panel. A pixel showing black is simply off, so blacks are absolute rather than dark grey.
For signage that buys two things. Contrast holds up in dim environments (hotel lobbies, cinemas, late-evening retail) where an LCD's backlight leaks through and flattens the image. And with no backlight assembly, the panel can be thin enough to mount flush into a fixture, or curved around a column.
The caveat matters more in signage than anywhere else. OLED pixels age with use, so an element that never moves (a logo in the corner, a price column, a fixed navigation bar) wears faster than the content around it and eventually ghosts permanently. That is burn-in, and signage is its worst case: the same layout, sixteen hours a day, for years. It is why most menu boards and wayfinding screens still use LCD, and why OLED earns its place on short-run, motion-heavy creative rather than static templates.
OLED also runs dimmer than commercial LCD, which rules it out of a sunlit window regardless of everything above.