Ідіти is the plural imperative, you use it either when talking to several people, or when talking to a person you don't know well.
Іди is a singular imperative, you use it when talking to one person you know well.
I.e. if you address someone with ви, use ідіти (or йдіть after vowels). If you address someone with ти, use іди (or йди after vowels).