It’s a fair question because I usually always hear “ selling out to stop the run” which of course means Strongly committing most or all of the defense to stopping the running game at all cost, which in this context, means pretty much forgetting about defending the pass, or significantly opening up their pass defense to focus on stopping the run. This can be achieved by blitzing more to stop the run, taking out DB's and subbing in more LB's and D-linemen, loading the box pre-snap, etc..
don’t blame me if it’s a run on sentence………I got it off Reddit.