Technical Debt is the build of inefficient, undocumented, poorly designed code. Miraheze's code has in the past been known to a mix of all three and code rewrites have been necessary to implement new features.
This a tag which is designed to track all technical debt related tasks - these will likely be created from code analysis from SonarCloud where the fix isn't easy, requires thought or is part of a larger issue.
Anything code related can be tagged as Technical Debt here, though it will primarily be Miraheze Extensions.