Parent & child relationship/field
The easiest example to explain is laid out like this, we have the following models:
In this context we are talking about cars, so the make would contain a list of car manufacturers (Subaru, Toyota, Ford, Chevy, ect). Thus the model would contain some options like WRX, GT86, Mustang, Corvette.
The way we would setup the sales model would be a oneWay relation to each make/model, and in the context of this request when we create a new "sale" we would select the make (say subaru) now when I go to select the model I should only be able to see/select the "WRX" model since it's made by Subaru.
I should not be able to select "Toyota" "Mustang" as that car doesn't exist.
See for more information: https://github.com/strapi/strapi/issues/8708