"Stalled" means that progress on the issue has stalled; it may be used in different ways in different projects.

It may be that a problem was reported, but has not been possible to reproduce reliably. Or it may be that it is agreed that current behaviour is undesirable, but nobody can see a way to fix it that doesn't break something else (typically backward compatibility). It may mean that there is no consensus on whether the behaviour is a bug or not. Or it may simply be that the fix is dependent on some other work that has not been completed.

The issue has not reached a state that it can be closed, but further progress is not anticipated unless some external factor changes or someone comes up with a new idea.