What does Special Defense do for your Pokemon?

When your Pokemon's Speacial Defense raise it mean it is stronger to it's power. Like when your Kadabra's Speacial Defense raise it will be stronger Psy moves.

Edit from lipantwort:

Special defence is a stat which decreases loss of hitpoints from special attacks. Almost every attack in Pokemon is either categoried as special or physical. When physical attacks are used, defence is one of the factors which determines how big the HP loss is. When special attacks are used, the special defence is a factor which determines how big the HP loss is.