Would Someone Explain/link me to an explanation of Synergy?

Lets say you use Captain Phasma as your leader. If you also used FO Tie Pilot, FO Stormtrooper, FO Officer and Kylo Ren you get to fully utilize the bonus of your leader, since the rate of bonus attacks increases if your team is FO.

However, if you just used whomever, you'd still benefit from her leader ability, just not as much.

Another example would be using Anakin, who gives Jedi's increased offense. If you then populated your team with all FO characters, you'd be losing out on his leader ability completely.

Synergy is all about putting teams together that Compliment each other. Using 5 strong characters is nice, but there will be times when synergy > strength.

I run Phasma (leader), Old Ben (debuffer), Tie Fighter Pilot (attacker), First Order Officer (support), and Poe (tank).
While it looks like i have all of the pieces, this team works because of synergy between factions and abilities.

The first to act is FOO or Poe. Both use a turn meter reducer ability on the enemy team, ensuring that my whole team gets to act first. Poe also taunts, blocking my team members from any attacks that make it through (in the event that an enemy resisted the turn meter reduction effects).
Tie Fighter Pilot takes his turn, and as a superior attacker, he kills the biggest threat on the enemy team. He is a squishy hero, but because of the turn meter reduction from FOO and Poe, and because of Poes taunt, TFP is safely protected and allowed to do his damage.
After TFP, Old Ben takes his turn and uses mind tricks. This ability blocks the enemy team so they cant use anything but basic attacks, it debuffs their offense so that when they do attack, it doesnt hurt, and it also removes 50% of their turn meter.
Now Phasma takes her turn (last on the team to act), and casts Fire at Will. This ability increases my whole teams turn meter, giving all of my heroes a second attack (the enemy still hasnt had any turns). It also provides the whole team with advantage (which increases their damage). With the advantage buff, both FOO and TFP put out very high damage to finish off 2 more of the enemy team. Now it is 2vs5 by the time the enemy team takes a turn.
To finish things off, Phasma gains increased attack from having advantage, and FOO uses his give ally a turn ability, which allows me to finish off the remainders.

This is synergy. It is how your heroes interact and effect each other and/or the enemy.

HTH

synergy:the interaction of elements that when combined produce a total effect that is greater than the sum of the individual elements

in RTS genre this usually means characters complimenting, enhancing each other through the use of their abilities, passives etc.