Solo mode is used to visit a place privately, so that other players won't interfere with your testing.
Solo mode is only accessible through either your own maps or shared maps. To enter Solo Mode, either go to "My Roblox" and click "Play Solo", or click on another player's place, and if the player has their place set to shared, you can visit their place without other players.
Unfortunately, most players keep their place CopyLocked, so that other players don't copy their place, or play through the place and find hidden secrets.
Be aware that some scripts may not work properly in Solo Mode.