Zelda Randomizer is a piece of software which used with a ROM, allows a player to switch up almost everything in the game- eg the cave you normally get the Wooden Sword from could be Level 9, a shop, a rupee cave, a take any road spot etc. The actual location of the caves can be switched around too- say for instance instead of bombing Spectacle Rock in the "normal" spot, you need to bomb somewhere along the top of the screen or even the other rock.
You can set it so monsters from dungeons appear in the over world and vise versa (think potential Blue Wizrobes on the start screen).
There's countless vids on YouTube of people doing letsplays or even tournaments. THAT can explain things even clearer.