"Stackable rings" refers to a type of jewelry that can be worn together on one finger. These rings are usually designed to fit together and create a layered or stacked appearance. Full definition
I liked this one in particular, because it comes with 5 colors of stackable rings which you can mix and match according to your outfit or preference that day. (popcornandpandas.com)
This gold stackable ring with name is also available in rose gold and silver, and you can mix together rings of different materials for a trendsetting look, or wear alone for that dainty and classic feel. (onecklace.com)