As Chris says the best thing you can do is watch some YouTube videos for an indepth explanation and then decide if they are something you need or want to use. Essentially they are sunglasses for your camera lens allowing you to decrease your shutterspeed to create motion blurr. Depending on if you are making video or still photos you use them differently. For still photos you can use them in higher light levels for long exposure photography to capture motion in running water for example.
I haven’t made any videos yet but from what I gather you use them to match your shutter speed to your frame rate which gives your video a more natural motion blurr. There are rules to follow. The shutter speed should be equal to double the frame rate (25fps = 1/50 second shutter speed or 30fps = 1/60 second shutter speed). Below is an example of the effect. Its called 180 degree rule if you want to Google it.
