The simplified TLDR is that ISO for the very first time did not approve a proposed C++ standard.

9 out of 22 countries voted "no", six would have been enough. These were Denmark, Finland, Iran, Italy, Japan, Luxembourg, Romania, Spain, and Sweden. In a addition and independently only 12 of the 20 participating members approved when 14 were needed.

The dispute is most likely because of a single feature called "contracts".

Very simplified these are conditions placed in a function's signature that are supposed to be checked at runtime.

The proposed implementation of this feature has been controversial from the start with prominent members of the community (Bjarne Stroustrup amongst them) expressing concerns.