Com Standing Wave Ratio?

Can someone explain a com radio's Standing Wave Ratio?

During our trip to Key West, our #1 KX155 got damp. I removed it, dried it
out, and just to be safe, swapped places with our #2 KX155. After several
hours of flying and just after take off from Champaign IL, we noticed that
when in altitude hold mode our Stec 60-2 would enter a slight dive when
transmitting on the #2 com (now in the top slot). This happened when in
either Nav or HDG mode and when coupled to either the VOR or the GPS. I
found the following on Stec's web site and am looking for a little
education.

Q: Where do I start when the aircraft either climbs or dives when the mike
button is pressed to transmit?
A: Over the years it has become customary to place the autopilot at the
bottom of the radio stack. Check the standing wave ratio on the comm.
radios, a high S.W.R will affect the altitude transducer. Make sure the coax
cables are routed away from autopilot wiring, and check for proper antenna
bonding.

Thanks as always,

Jim