Scandinavian


assuming you're speaking from the balck pieces, here's a sample.
Black plays to restrict the d4 pawn and eventually once castled, perhaps undermine white's center with c5. Let me know if there's any more specific questions you have that I could explain.

By playing 1...d5 (White if they hope for any opening advantage must take the pawn) which drives the game into positions somewhat of Black's choosing. (i.e and thwarting White usual main line prep) Because of this, it's the bane of many 1.e4 players.
As someone who plays the Center Counter Defense (Scandinavian to Euro peeps ) It always amazes me how much White will fight against playing the main lines, when they are indeed White's best moves... so often resorting to the Tenssion Gambit, the ICBM or the BDG as a means to deal with 1...d5.
The Center Counter Defense isn't for everyone. It's a counter punching defense. By violating an opening principle (an early deployment of the Queen) White gains a tempo kicking it from d5. Because of this ..... Black essentially has to play defense for the first 10 moves or so, then attempts to counter attack when he/she has caught up in development. Again the trade off for violating that opening principle "usually" results in the practical advantage of knowing the resulting positions.
Many see this strategy as suspect ?! But.... how many defenses do this or violate some other opening principle and are considered sound? Alekhine's defense comes to mind, or the French on how it locks in the LSB, or the Advance Caro-Kann lines where you play c6, then c5 wasting a tempo.
Another driving factor is by playing the Mieses Var 3...Qa5 Black can get an improved Classical Caro-Kann while avoiding various dangerous White divergences, like the sharp Advance Variation, or the Panov attack
I'll leave the reader with the latest popular swindle attempt by White using a line in the Mieses-Kotrc var...

Another driving factor is by playing the Mieses Var 3...Qa5 Black can get an improved Classical Caro-Kann while avoiding various dangerous White divergences, like the sharp Advance Variation, or the Panov attack
Exactly.
The main line of the Scandinavian leads to a typical Caro-Kann type Pawn structure...
... but by playing a Scandi move-order (instead of a Caro move-order) the Black player avoids some of the more forceful alternatives for White such as the Advance variation, the Panov-Botvinnik or Tal's quick-development lines.