GitHub: What is a "wip" branch?
On GitHub, pull requests are prefixed by [WIP] to indicate that the pull requestor
- has not yet finished his work on the code (thus, work in progress), but
- looks for have some initial feedback (early-pull strategy), and
- wants to use the continuous integration infrastructure of the project. For instance, GitHub Actions, TravisCI, CodeCov, and codacy.
More motivation for WIP pull requests is written by @ben straub at https://ben.straub.cc/2015/04/02/wip-pull-request/.
New Since Februrary 2019, GitHub offers draft pull requests, which make WIP more explicit: https://github.blog/2019-02-14-introducing-draft-pull-requests/
Conventionally, "wip" stands for "work in progress".
Literally it means Work In Progress (WIP) as previous answers correctly point out. However, it does not occur only on GitHub, but can happen also on any other competitive platform, e.g. Bitbucket, GitLab etc.
It can be the case also with your Git (the VCS, not GitHub, GitLab, etc. - it's not the same) on local machine. In situations when you would like to save your progress on current branch and move to another it can be helpful in order not to lose your uncommited changes. In such a way you'd like to use use git stash. Then you will see the WIP... as one of the branches in Git Bash/GUI.
If you would submit this branch then of course you can see it in the project on Bitbucket/GitHub/GitLab too. Therefore, it is not only for push Pull Requests (PRs), but might be accidentally/on purpose pushed too.