“Ping pong” refers to reconciling the differences between a House-passed bill and a Senate-passed bill by amendments between the chambers, rather than forming a conference committee.
The formal term is “messages between the Houses” and is much simpler than a conference. However, a conference is a more structured way to reach an agreement and often leads to the chance of building support for the final passage of the bill.
The New Republic describes the ping pond process:
With ping-ponging, the chambers send legislation back and forth to one another until they finally have an agreed-upon version of the bill.
But even ping-ponging can take different forms and some people use the term generically to refer to any informal negotiations.