Frogs are the things that you use to save in Mother 3. They can be found in many different places, with many different looks. They usually are just hopping around, but one is in a wheelchair, one is hanging from a balloon, one is flying on butterfly wings, and many other strange appearances. They don't all do the same thing, certain special frogs do not let you withdraw/deposit Dragon power, or DP. There are at least two frogs in every chapter except Chapter 6, which is the shortest chapter in the game.
Frogs usually leave some sort of remark before you save your game. They also say "*hop* save your game?" before you choose to save it. There is a frog early in the game that "Really recommends" you save your game after a challenging part, followed by another challenging part, with only the frog as a break. Frogs usually appear after particularly challenging parts, or after finishing a chapter.