From the Super Mario Wiki
A Super Guide Block
from New Super Mario Bros. U
A Super Guide Block is a special green Exclamation Mark Block that appears in New Super Mario Bros. Wii and New Super Mario Bros. U. It will appear when the player has lost eight lives (in New Super Mario Bros. Wii) or five lives (in New Super Mario Bros. U) consecutively in the same level. If Mario hits the block, the Super Guide mode will be activated, Luigi will appear and the computer within the game controls him to clear the stage in place of the player. If the player presses on , the player may choose to deactivate the Super Guide mode and take control of Luigi. However, if the level is beaten, it won't count as an actual win. Items collected do not count as well.
Note that if a Super Guide block appears in New Super Mario Bros. Wii, stars on the save file will no longer be shiny. However, in New Super Mario Bros. U, stars on the save file will stay shiny even if the block appears, as long as the Super Guide is not used. Also, the block only appears in single player mode, even if the second or third player are Toads.