Unless you mean 'responsive' like 'works on mobile'. Which seems kind of a moot point considering we're talking about desktop applications.
In theory it should be applications automatically showing and hiding features based on the available allocated area so that you could have (on desktop at least) multiple applications visible at the same time and have a dynamic view of their content's details just by resizing them. Which is a good idea.
In practice however it ends up as a band-aid for self-inflicted deficiencies.
It's useful because we see more and more 'convergent devices' like the librem 5, pr a raspberry pi with a 10' screen