Checkpoint Flag

The Checkpoint Flag (also known as the Midway Flag or Halfway Flag) is a recurring object that signifies a checkpoint in several Mario games. They are similar to the Midway Gates from Super Mario World. If Mario or Luigi lose a life after passing a Checkpoint Flag, they will restart at the point in the level in which the Checkpoint Flag is positioned. If Mario is Small Mario, the flag will always restore him to Super Mario.

New Super Mario Bros.
Checkpoint Flags first appeared in New Super Mario Bros. as white flags with a picture of a Mushroom on them, though not all courses had them. They're not stuck on the ground, and appear suddenly when Mario reaches a checkpoint zone.

New Super Mario Bros. Wii
Checkpoint Flags reappeared once again in New Super Mario Bros. Wii. In this game, they appear as black flags with Bowser's symbol on them. When Mario, Luigi or one of the Toads touches one, the Bowser symbol will be replaced by the symbol for whatever character touched it. The player(s) also receive(s) 2000 points. Additionally, if Mario is currently Small Mario, he will become Super Mario when the Checkpoint Flag is activated.

Super Mario Galaxy 2
They reappear again in Super Mario Galaxy 2 and feature almost the same appearance and function as in New Super Mario Bros. Wii. Like in that game, when Mario or Luigi touches a flag, the Bowser symbol will change to their respective symbol (a "M" or "L" symbol). Depending on which galaxy the player is in, there may be anywhere from one to several Checkpoint Flags positioned between the beginning and the end of the level. Some levels, such as "The Perfect Run" in the Grandmaster Galaxy, may not feature any Checkpoint Flags at all.

Super Mario 3D Land
Checkpoint Flags, having the same design as the ones from Super Mario Galaxy 2, they appear in the Nintendo 3DS game Super Mario 3D Land. In this game, the flags turn Small Mario to Super Mario, like before. They serve the same purpose they did in the previous game and like before, some stages like World 2-1 may not have Checkpoint Flags at all.

New Super Mario Bros. 2
Checkpoint Flags reappeared again in New Super Mario Bros. 2 similar to how they appeared in New Super Mario Bros. Wii, giving the player 2000 points and replacing the Bowser symbol with either a "M" or "L" symbol.

Trivia

 * In Super Mario Galaxy 2, Checkpoint Flags don't always have to be activated by Mario or Luigi walking into them; they can also be activated by the Co-Star Luma or by Yoshi's tongue.