Players win games, teams win championships.
If you read the previous page, you have a basic idea about how to pick a unit. But, there's more to it than that. An army isn't a group of randomly collected units, it's a set of units that work together to accomplish a task.
No unit exists in a vacumn. The idea I want to stress here is that when you're planning your army, you need to think about how all the pieces will work together, not just how each unit will do on its own.
Some examples to consider
You have an army that's pretty much dedicated to getting troops into assault. What is the typical reaction of an opponent going to be? If they're not playing an equally powerful assault army, they're probably going to try and sit back and shoot you. The further away they sit, the more time they're going to have to shoot your troops, and the longer it's going to take you to get into assaulting distance.
How can you convince your opponent not to just sit as far back as they can? Introduce a secondary threat, one that punishes them for hanging too far back. The indirect fire on a basilisk, or a defiler, for example, can hit just about anywhere on the table, except close to the artillery piece. Your opponent is then forced to choose between sitting far away and being shelled, or getting close enough to avoid the shelling, but having to face your assault troops. This dilemma that you pose your opponent works because your army list was designed with the synergy of long-range fire and close-combat troops in mind.
Sometimes, synergy is found in selecting an option that may not be the most optimal when taken by itself. For example, an independent character is afforded some measure of protection against shooting due to being an independent character. As long as he's not the closest target to the enemy, he's safe. Now, giving this character a jump-pack or jet-bike of some sort may initially seem like a good way to get the character into combat faster. But, without troops that can keep up with the character, what it really does is provide your opponent an easy opportunity to snipe your character. The leader is not synergistic with the army. Either a unit needs to be included to advance with the boss, or the boss can trade in their jump-pack for a solid pair of boots and stay with the troops.
Synergy is the key to an army that functions like a team, rather than like a group of units. It is about planning, while making the army list, how the units will ideally support each other, how they will move around the battlefield, and how they will force the opponent to react to your moves.
Synergy inside units
Synergy can also be found when designing units. A unit that has many options for what equipment it can take needs to make sure that the weapons they choose function in a complementary fashion.
For example, a unit that can take two anti-infantry weapons, a flamer and a heavy bolter, may not want to do this. The heavy bolter has a good long-range, and requires the firer to remain stationary in order to use it. The flamer is extremely short-ranged, and so demands that the bearer get close to their enemies. Although these two weapons are both seeking to target infantry, their use is not synergistic. One or the other will most likely go unused.