Which tournament were you watching? An MLG?
As a quick rundown of the game: SC2 is heavily based on the player's economies. Whoever has more workers probably has more income. Whoever has more bases probably has more workers. More income means the player is more likely to win later, when he can spend that money. If someone has less income, they likely have a bigger army, which means they have more of a chance of winning the game right now, before the other player can use their larger income to make more units.
Also, I'd argue that traditional sports are just as hard to understand from an un-introduced spectator's point of view.
Looking at a game of football the only moments that stand out as important to me are either an amazing running play, an amazing throw, and a touchdown. These are all parts of the offensive game, not the defensive side. And that's not even getting into the aspect of the different plays, either.
Looking at hockey, or soccer, the only big moments for someone not "in the know" about the game are basically when a team scores.
There's a massive amount of depth to these games, but I can't see any of it. I've played ball hockey, soccer, and football for fun and never learned a damn thing about the depth of the game. And watching a game of any of these on TV would never help me understand anything about the depths of the game if I listened to the commentary. And thee are games we grow up playing as children and such.
There are people out there who've never played a single game of starcraft in their life who could talk with a player about builds, the metagame, and deeper topics in general.
I'd highly suggest paying close attention to the commentators. They go out of their way to explain intricacies in the game (especially if an unusual situation arises) and will often discuss some fundamental important things about the game. Day9 is famous for being the spokesperson for introducing people to the game.