The average date of the latest (up to) 50 commits to this library's source code repository default branch on GitHub.
A recent date often is a good indication that the project is actively maintained.
However, direct comparisons of this metric across libraries are usually not very helpful as development flows can have huge variance - for example some maintainers might be making lots of small commits, others might make single large ones or squash them when merging a pull request, which has a very direct influence on this metric.
Additionally, a library that is seeing a lot of commits might be undergoing larger changes, which could potentially decrease backwards-compatibility.