A Filter is a named object, which uses item properties to include ("pass") some items, but exclude ("fail") others.
Filter objects work by examining the properties of an Item. So one filter could look at the colour of an Item, and only "pass" those that are red. (In the same way as a camera can use coloured glass to filter the images of the photographs it takes.)
Filter objects can be applied to overlays to control the visibility of the items in that overlay. Only items that pass are drawn. Items that fail the filter are not drawn, so you can make them temporarily invisible without having to delete them. In addition, many SIS methods, e.g. Scan, accept a Filter as an argument.