"Total time away from gate", "Longest time away from gate" - what does this mean?
The US regulations have the tarmac delay rules, which apply to extended delays where passengers are left sitting on the tarmac. This followed a variety of incidents where passengers were kept on board non-moving aircraft for long periods of time, sometimes without access to food and water. The rules are a bit complicated with various exceptions, but broadly: 


  How long can an airline keep me on a DEPARTING flight before the
  airline is required to start moving the airplane to a location where
  passengers can safely get off?
  
  For flights departing from a U.S. airport, airlines are required to begin to move the airplane to a location where passengers can
  safely get off before 3 hours for domestic flights and 4 hours for
  international flights.


So yes, a flight can potentially have multiple tarmac delays. For example, a flight could potentially leave the gate on time, sit on the tarmac for 2.5 hours, return to the gate (to allow passengers a chance to get off, as is required), leave the gate, wait another hour, be cancelled, and return to the gate. The airline would report both all the time the plane was away from the gate (2.5 + 1 = 3.5 hours) and the longest period the plane was away from the gate (2.5 hours). 


  What is a "single time"? A single time period out of multiple ones?


Yes. If the flight sits waiting on the tarmac for more than one period, the "single time" is whichever period was the longest. 


  Why would a plane be away from the gate multiple times? i.e. leave the
  gate, return to the/a gate, leave again and return again?


It could happen for a number of reasons. One would be that the plane returned to the gate to comply with the rule and allow passengers an opportunity to get off after 3-4 hours delay. The plane might return to the gate for a maintenance issue. They could be waiting out bad weather or a situation where an airport is not accepting flights. Most airlines seek to avoid this, but it can happen, and the reporting requirement is intended to capture data about how long it takes when it does.


  Why would a diverted plane be on the tarmac multiple times at the same airport?


The same reasons. I had this happen once due to a tornado. The flight diverted, we went to a gate, we went back to the tarmac to wait for better weather, the flight was cancelled, so we went back to a gate again. It's an unusual situation, but it can happen.