Star Coin Sign

"You need 5 Star Coins to pass this area. Do you want to spend the Star Coins?"

- Star Coin Sign

A Star Coin Sign is a wooden sign blocking a path on the world maps in New Super Mario Bros. and New Super Mario Bros. 2. They cost five Star Coins to remove, and doing so opens up either a Toad House or an alternate level; the player will receive a prompt to save after opening one.

In New Super Mario Bros. 2, a different Star Coin Sign blocks the path to a Yellow Toad House in World Star; this one only appears after collecting all the Star Coins in the game, including World Star's Moon Coins, and costs nine Star Coins to remove.

While they do not appear in New Super Mario Bros. Wii, similar gates are located in World 9 of that game, and can only be unlocked by collecting every Star Coin from the world corresponding to that level (i.e. World 1 opens 9-1, etc.);

Reference
Cartello Monete Stella