Dubbel, tripel and quadrupel are beer styles from the Belgian and Dutch monastic tradition. The names sound like arithmetic, but that's not how it works.
Dubbel
A dubbel is dark, malty and sweetish, with notes of caramel and dried fruit. It usually sits around 6 to 8%. The best known is Westmalle Dubbel.
Tripel
A tripel, by contrast, is golden, powerful and spicy, often between 8 and 10%. In 1934 Westmalle released a strong golden beer that's often seen as the first tripel. Well-known tripels are Westmalle Tripel and Tripel Karmeliet.
Quadrupel
A quadrupel is even fuller and darker, often 10% or more. In the early 1990s La Trappe was the first brewery to actually call a beer 'Quadrupel'. La Trappe Quadrupel.
Where do the names come from?
Nobody knows for sure. One well-known explanation is that brewers used to mark casks with crosses to show strength: X, XX, XXX. Another is that it referred to the amount of malt. Either way: a tripel isn't three times as strong as a regular beer, and you don't have to pay three times as much for it either.
These beers are stronger than a lager, so take it easy: smaller glasses and some water in between.