Decomb

If you see combing or mouse teeth (horizontal lines) in the preview images, you should apply deinterlacing.

But deinterlacing is slow and reduces picture quality. And a lot of the time it isn't necessary. And it's not always easy to know a video is interlaced just from looking at previews. And then you waste a bunch of time deinterlacing frames in interlaced video that don't even show combing, slowing down your encode.

Wouldn't it be nice you could tell HandBrake to just deinterlace a frame if it's interlaced?

That's what Decomb does. It should be safe to leave it on all the time with a setting of Default. By selecting Custom, a text field will appear where you can directly set the decomb filter parameters.