This is a display base or stand.
Adapted from the GoodSmile website:
Colorful and cute heart bases for your Nendoroids!
The Nendoroid More series was created to make Nendoroids even more enjoyable! The newest item in the Nendoroid More series is a collection of heart-shaped display bases for your favorite Nendoroids! In addition to eight simple color variations, three glittered bases are available as well. Be sure to grab a few for your Nendoroid collection!
[Size]: Approximately 7.2cm x 6.5cm x 1cm
[Materials]: ABS, PP