What is Wii Fit?

My husband just gave me a Wii for my birthday and included Wii Sports, Wii games and some other games. Someone suggested I get Wii Fit and I’m curious about what it is. If it appeals, is it something I can get just about anywhere? Does it require anything special with the system? I know nothing about video games, so my apologies if it seems like something most 7 year olds would know. :~)