Apple has a ‘Smart Stack’ of widgets for its own apps, but it’s easy to make your own for any apps. Simply drag a widget on top of any other widget of the same size, and they’ll stack. I think seven is the max for each stack.
In my example here, I drop a Lumy widget on top of a widget for Apollo, a superb Reddit client.
To remove widgets from a stack, get into edit mode, tap the stack for a list of its widgets, then swipe left on one.