With every game you play in Starcraft 2 you must scout your enemy.
Scouting is seeing what kind of strategy your opponent is going for.
They could be going for a rush that you are not prepared for so once they send their early army over you are screwed.
That is why scouting is very important.
If you do not know what your enemy has then how do you expect to counter that.
Just as an example lets say you are Terran facing Protoss.
So you go the standard build of 3 maybe 4 barracks producing marines and marauders because this works well against Protoss players.
You even decide to get the main upgrades which are stim pack and marine shields which I believe is s must when going for this build.
You might even throw some medivacs into the mix because these units are primo together.
The whole time you never really scouted as you feel confident to just go over and attack.
But then once you get over there you find the units that own marines and marauders which are indeed protoss colossi which sweep over your units fast causing you to GG the game.
If you scouted then you would know to thrown in vikings which are the best answer to colossi.
Of course if they massed up on their colossi you would need as many vikings you can.
Without scouting you are taking a shot in the dark.
You should just scout every game so that you know what to go for and better yet get your timing down.
If they decide to tech up to units that go well against yours you must rush them to put the pressure on them.
If they rush you then you need to be a "turtle player" and defend until they ran out of units leaving them wide open.
But if they play defensive then you should tech up.
Think of these strategies like rock-paper-scissors since each one loses to one and wins against the other.