Randomizer Styles
Randomizer styles (aka randomizers) are objects that are not tied to one gridslot, because they can affect multiple gridslots at once. A single map can have up to 8 active randomizers. For more info about the panels which allow you to edit randomizers, visit this page.
The purpose of a randomizer is to add detail to the background layer without requiring each seperate gridslot to be given a detail image manually. The background layer info page explains that this layer can exist in one of 3 types. The simple type only uses one image for this layer, while the detail type allows to place another image on top of it. The randomizer type can have a second image as well, but instead of this image being a fixed sprite with certain fixed stats, every aspect of this image is controlled by a randomizer style. When editing a randomizer background layer, one doesn't get to choose a second image, instead one can select which randomizer style should be affecting this layer.
Imagine an indoors level, the background is a certain floor type. To add details to this floor, one could edit each individual gridslot which might take a fair bit of time if the map is large. In such a case it's more interesting to use a randomizer. Basically a randomizer controls the odds that a certain number of tiles, randomly chosen from all the tiles it applies to, receives a randomly chosen detail image whose various stats (hue, brightness, offsets) are randomized. Every time a level with a randomizer active is loaded, this randomizer will generate a new set of detail images causing this level to look slightly different each time.
For a detailed overview of which randomizer stats can be edited, visit this page. In general, stats come with editable minimum and maximum values between which a random value is chosen for each affected slot.