It is used in two ways. It is used if a person is fairly sure of something, but there is a slight chance they are incorrect. "If I'm not mistaken, she was married before this." It is also used if a person is absolutely sure of something and is using it for emphasis. "If I'm not mistaken, the value of pi is 3.14159." Personally, I use the phrase when I am more than 92.7% sure of something. ;-)

Probably all English speakers have used the expression at one time or another, if I'm not mistaken. It is a parenthetical.