It's basically something that congratulates you on your completion of Portal. Plus, one of the reasons they did it is because they knew that since it was part of the Orange Box, people were going to jump at Team Fortress 2 and The Half Life games, so they wanted to start something new, Portal.
But they didn't know how to get people to love it. So they threw in a female voice, no other humans in sight, and a song at the end. And anyway, if you didn't play the game, you shouldn't be istening to it anyway because the entirety of the song is, as, Kaekun said, reflecting off the events which happened in the game.