A collection of art pieces from Pixiv artists that have received one of the highest evaluations from that community, that is, being bookmarked by over 1000 members. These pictures are usually denoted with "1000users入り" tags. For example 東方Project1000users入り are posts belong to the Touhou series, オリジナル1000users入り for original, VOCALOID1000users入り for Vocaloid, et cetera. Occasionally, there will be images that have achieved "5000users入り", "10000users入り" and even "50000users入り" too.
You can look for series or characters you like that belong to this pool by performing pool:2778 abc_xyz search (replace abc_xyz with what you want).
For any group of images that qualify, only the first page in its respective pool is included, and the pool name will be mentioned in this description: