Full Idea
In standard ZFC ('Zermelo-Fraenkel with Choice') set theory we deal merely with pure sets, not with additional urelements.
Clarification
The prefix 'ur' means 'basic'
Gist of Idea
ZFC set theory has only 'pure' sets, without 'urelements'
Source
E Reck / M Price (Structures and Structuralism in Phil of Maths [2000], §2)
A Reaction
The 'urelements' would the actual objects that are members of the sets, be they physical or abstract. This idea is crucial to understanding philosophy of mathematics, and especially logicism. Must the sets exist, just as the urelements do?