Check under performance information.
PNG: Ping -- the time it takes for packet to be sent to the server and back.
RTT: Round Trip (Delay) Time -- calculation-wise, this would be the same as ping, but from what I understand from the support site, my guess would be that this is number is measured with a weighted moving average while PNG is reported as instantaneous RTT.
IND: Interpolation Delay -- how long it takes for YOUR client to "smooth" the information it gets from the server. EG, the game smoothly transitions a change in a characters position and velocity so it doesn't appear jumpy.
To answer some of the relationship questions based on my assumption about RTT. I haven't paid much attention to how the numbers relate to Overwatch specific experiences, so this is more theoretical than empirical.
PNG>RTT: Your ping is higher than your "average ping". If it's much greater, you're probably having a ping spike.
RTT>PNG: Your ping is lower than your "average ping." You might be recovering from a ping spike. If RTT is higher than the ping you think you should have--especially on a consistent basis--there might be connection or network traffic issues ANYWHERE between your PC and the server (that means on your home network, between your network and ISP, from the ISP to the back-bone to Blizzard's server).
For IND, I'm not sure what actual number being reported is, but normally interpolation is affected by when you get an "update"--so Ping/RTT, the client-side tick-rate and your FPS. If IND is high, you're getting a less accurate view of the game--things are being shown to you later than they are actually happening. If your IND is abnormally high and your ping and rtt are normal, you might have a performance issue.