Most Belgian style ales are fermented with one yeast strain. Once fermentation and maturation have completed the beer is bottled. Belgian brewers add a mixture of priming solution along with some more of the same yeast and allow the beer to referment and carbonate in the bottle.
The reason they add the same yeast at bottling time is because most of the original yeast would have flocculated and would be too tired to eat the new sugar and carbonate the beer. The new yeast is fresh, it can ferment the priming solution and since the beer is bottled the CO2 will dissolve in it, creating carbonation.
If you see “bottle conditioned” on a beer label, it means fresh yeast was added during bottling to ensure carbonation.