In Sessionwire's case, "round trip time" refers to the time it takes for a network signal to traverse the internet from the source, to the destination, and then back to the source again. It is an indicator of network health - high RTTs can result in significant audible and visual delays in the Sessionwire app.
RTT, however, is NOT the amount of time it takes for audio to make the round trip during a Sessionwire call, as there are other factors that affect audio round trip time including DAW buffer size and hardware audio signal path on one or both sides of the connection.
In order to ensure RTT is as low as possible, Sessionwire recommends using an ethernet connection with wi-fi turned off. This also ensures the stability of the connection is consistent as wi-fi connections can suffer from packet loss, which can cause audible dropouts in streamed audio.