In
Sanskrit word "kumbhaka" means breath suspension. Quite common
jargon heard in any yoga training hall.
Kumbhaka is connected with marshalled pranayama practice and is used by instructor to command aspirants to hold their breath for a while. It depends firmly on your "yogic experience level" what the concept "for a while" really means; usually one does kumbhaka for about 30 seconds, but it is not unusual to do kumbhaka for 2 minutes or more when it comes to experienced yogis.
It is believed that pranayama practice and especially kumbhaka-practices will purify aspirant's body and mind. Kumbhaka-practice is quite similar to more commonly known buddhist breath-observing meditation. However, buddhist meditation does is not concerned about physical progress and should not be muddled with pranayama.
Ashtanga lineage of yogic practice hold Kumbhaka-practice most important style of pranayama (sometimes gurus have been heard to say that kumbhaka-pranayama and asanas are more than enough for weak souled westeners).