What are "backstab" receptacles/outlets?
If I understand correctly, &quot;backstabbed&quot; receptacles are such that:

Have holes in the wall-facing side,
into which you push a wire, and
the wire is held by the pressure of metal plate pushing against it - a bit like a spring.

You may also find them referred to as &quot;back-wire&quot; or &quot;quick-wire&quot; receptacles.
important: &quot;backstabbed&quot; receptables are notorious for low reliability, and specifically, the tendency to overheat, arc and melt. Many suggest avoiding or replacing them.
Illustration of how such holes may look like from the back:

and here is a comparison between a clamping-based (on the right) and a &quot;back-stab&quot; mechanism (on the left):

There's a longer treatment of back-stabbing on Quora.com:
What does it mean when an electrical outlet is backstabbed