Fundamentally, LCD displays like this work by controlling which pixels are dark or light by having polarised light coming from behind (the yellow glow) and then blocking it by switching the polarisation of liquid crystal at a pixel-by-pixel level.

When the liquid crystals are aligned with the light the light gets through to your eyes, when unaligned the light is blocked.

As viewing angle increases, increasing amounts of light leaks though ‘dark’ pixels, because the liquid crystal is no longer effectively blocking it at that angle.

That’s my simplistic understanding