When a person has several orgasms, in a row, without any, or minimal, time in between achieving the orgasms.
Men who experience 'dry orgasms' also report having multiple orgasm, mainly due to the lack of need for a 'refractory period.
Most men do NOT have multiple orgasms, due to the need to 'reload'. This is the state where they body needs to replenish itself, from the previous orgasm.
However, it is possible, for men who have dry orgasms, or who can experience an orgasm, without ejaculation, to experience multiple incidents.
In addition, those who refrain from ejaculation for a period of time, can also not only experience multiple orgasms, but the force of ejaculate, and the associated pleasure is reportedly more intense and forceful.
There are other techniques one can use, to achieve multiple orgasms, however some carry some risks, if practised too often.