A Deinterlacer is a TV processor that converts interlaced video signals (old broadcast format) into progressive video, which modern flat-screen TVs use.
Interlaced video shows odd and even lines separately, causing jagged edges or flicker. The deinterlacer blends them into a smooth, full frame.
For example, Kenyan free-to-air channels that still use interlaced signals look clearer and steadier on TVs with a good deinterlacer.
See also: Deinterlacing, Progressive Scan, Upscaling, free-to-air