This form is acheived by Goku in the hyperbolic time chamber after Cell is defeated and before the World tournament takes place. Vegeta also learned it at the same time as Goku. Gohan never mastered this level of super saiyan, saying it was not worth mastering, now that he had mastered the full power super saiyan. Trunks had not mastered this form, neither has Goten.
The appearence of Multi super saiyan is the hair stands straight up, and turns pure gold, and muscle increases rapidly.