CRT shaders emulate the display and output of old-fashioned cathode ray tube televisions. This includes visual distortion, smearing, and scanlines.
It isn’t 100% accurate to the old-fashioned television sets some of these retro consoles would’ve been commonly used with, but it does add a hint of immersion.
CRT shaders aren’t generally enabled by default, with emulators like RetroArch it’s an option you can select on individual games or system cores.