A range relationship basically means that a starting value is determined ( either by data entry or via a calculation ) and an ending value is determined ( again either by data entry or a calculation ). FileMaker adds a new level to ranged relationships with the new comparison operators and multiple levels of matching criteria. Ranges can be built on almost any type of data but date range would likely be the most common. You will also see ranging relationships for numbers, time and even text. The matching records of a relationship range can be displayed in a portal or used to build a found set by using the GTRR command.
Here you can see that the FileMaker child key data has two layers. The appointment date needs to be greater than then start date and less than the end date. You can see how multiple layers are very handy for relationships that range.