The Scandinavian Defense is one of the oldest recorded openings in chess. From the very first move, black looks to undermine the center control of white after the common e4 move by white.
Nine times out of ten white will catpure the pawn on d5 and I recommend that you spend the majority of your time learning the scandinavian defense, assuming that white captures the pawn on d5.
Black then has two options. The first option is the main line which is to recapture the pawn on d5 with his queen. Then after white play Nf3, black can decide to bring his queen to a5 or back to d6. It is considered bad play to retreat back to d8 since the queen just came from that square. You shouldn't play the scandinavian if you are looking to merely retreat your pieces when they could stay active.
The other response black has is to play Nf6 and not recapture right away on d5. This is done for two reasons. One black is giving up material to build up a stronger center and get better development from their minor pieces. The second reason is that the main line is for white to give back the material and black will have a better developed plan than they would with their queen exposed.
Viswanathan Anand vs Joel Lautier
Credit Suisse, Biel SUI 1997
Scandinavian Defense: Main Lines. Mieses Variation
1.e4 d5 2.exd5 Qxd5 3.Nc3 Qa5 4.d4 Nf6 5.Nf3 c6 6.Bc4 Bf5
7.Ne5 e6 8.g4 Bg6 9.h4 Nbd7 10.Nxd7 Nxd7 11.h5 Be4 12.Rh3 Bg2
13.Re3 Nb6 14.Bd3 Nd5 15.f3 Bb4 16.Kf2 Bxc3 17.bxc3 Qxc3
18.Rb1 Qxd4 19.Rxb7 Rd8 20.h6 gxh6 21.Bg6 Ne7 22.Qxd4 Rxd4
23.Rd3 Rd8 24.Rxd8+ Kxd8 25.Bd3 1-0