It's a code that lets you play as Samus wearing her "Bikini" suit instead of her usual Power Suit. In addition, Samus will already have most of her upgrades (Missile tanks, Wave Beam, Long Beam, Screw Attack, Bombs, Varia Suit, etc.). I think the only one that's missing is the Ice Beam, but I'm not sure.
The actual code itself is as follows:
JUSTIN BAILEY ______ ______
There is also a variation of the code which places Samus in a different location if the dashes are replaced with spaces.
If you put dashes halfway across the bottom row and leave the 2nd half blank, you get the ice beam.