Shepard's pre-service history tells about Shepard's actions before the start of the game. Each one carries it's own morality weight and each one opens up a specific optional mission for each specific history, depending on the one that you chose.
This Pre-Service history starts you off on the renegade side of the morality scale. It means that Shepard was born on Earth and was a part of the metropolis community for a time. before joining the Alliance at age 18. It also means that he/she did some things there that isn't exactly acceptable in the Alliance military. The selection of this pre-service history gives you access to the mission UNC: Old Friends, in which an unlikely character from Shepard's past has returned to stir up trouble.
This Pre-Service History starts you off on the paragon side of the morality scale. It means that Shepard followed in her/his parent's footsteps and, after living aboard starships all her/his life, joined the Alliance the first moment she/he could. This selection opens up the UNC: Old, Unhappy, Far-Off Things mission, in which an old friend of Shepard's mother needs help.
This Pre-Service History starts you off in the middle of the morality scale. It means that Shepard lived in a colony on the planet Mindoir before it was attacked by slavers who killed her/his parents in the cross-fire. She/He was saved by a passing Alliance patrol and joined the Alliance the very first moment she/he could. The selection of this pre-service history opens the UNC: I Remember Me mission in which a survivor from the raid of Mindoir is unstable and has a gun. You have to stop her and give her a sedative.