Backlit means lit from behind, so the front of the subject stays dark. A backlit figure, backlit trees, shoot it backlit.
In photography it produces silhouettes and glowing edges, and it is difficult to expose for — the camera sees the bright background and darkens everything else. Rim light is the halo it puts round a subject's edge. In screens and keyboards, backlit means lit from behind so it can be read in the dark: a backlit display, backlit keys. Note backlighting, the noun, and front-lit and side-lit.
1adjectivebacklitLit from behind, so the front stays dark.
“Every figure in the shot is backlit and unrecognisable.”
2adjectivereadableLit from behind so as to be readable in the dark.
“The keyboard is backlit, which is the only reason he can type at night.”
backlit is a lighting arrangement.
The word is back plus lit, a straightforward compound from the language of photography and stage lighting. It appeared in the twentieth century alongside sidelit and underlit, all built the same way.
From back + lit.
Etymology from Wiktionary, CC BY-SA
Practised in Light & Shadow, Shapes & Lines