Issues

Issues, similar to Pull Requests, have old names they have somewhat outgrown. Traditionally, Issues would be things like bug reports, literal issues with a program. Currently, issues are much closer to anything that is an "issue" with how the program functions. This includes bug reports, but also situations where expectations don't meet reality, making a suggestion, proposing  a change in method, rewriting something, anything that can be boiled down to "the program might be doing x wrong, and we should change that." Issues are not the place for doing change, they are the place for identifying where change is needed. If you want to make changes, you do a Pull Request (which are often created to resolve the needs of a related Issue).

Each Forge repository has its own set of Issues, which are all considered Open (the change has not happened, the Issue is still here) and Closed (something has changed, or the Issue is not to be addressed). These will have a numerical ID, leading to specific numbers sometimes becoming infamous (such as Continuwuity Issue 779). These issues may end up having people assigned to resolve them, long threads of replies to narrow them down, be put onto Project boards, mentioned in commits, closed by Pull Requests, put into Milestones (goals to reach), and even have due dates. Issues themselves represent a single unit of what needs to be done to the program.

Issues, however, are still largely used for bug reports. Bug reports are the most distilled version of "something must change" after all, and all the tools mentioned about can help bugs get squashed.