Reznors in Super Mario World

There are 96 exits in Super Mario World. The star signifies that you have done all of the levels.

More recent Mario games (like New Super Mario Bros. DS, and Super Mario Bros. Wii) have multiple stars available for completing different goals (beating the game. collecting all the coins, finding all the exits...), but Super Mario World only has one star.

Interesting fact:
In the Japanese Super Mario Bros. the number of completed levels is written in Japanese, unless you get to 96. At 96, it just displays the number "96" in our number system. (no star)

You have to break (touch) the rope at the level's exit to earn stars. The higher it is when you break it, the more stars you get — up to 50. Getting all 50 gets you three bonus extra lives.

The white horizontal bar between the two vertical striped bars is the 'rope' to which I refer.

Once you have acquired 100 red stars, you enter a bonus game where you can earn extra lives.

The minigame starts with 8 squares that had changing icons on them (the one in center does not change). The 8 squares around the center rotate around the middle square. The objective is to hit the moving squares to make them stop changing their icons. Once you are done, you get an extra life for every row of similar icons.