In most cases exactly how Manabarbs ability works doesn’t matter. Players will end up taking a point of damage each time players tap an individual land for one or more mana. However this ability is a triggered ability that does use the stack and knowing this can be useful under the right circumstances.

Mana abilities do not use the stack, so when you tap a land for mana you’ll get that mana immediately and then the trigger from Manabarb’s ability will go on the stack. At this point you can cast instants and use activated abilities using the mana you just generated. This allows you to resolve something before the Manabarbs ability resolves. For example, you can tap a Mountain for red mana, put this trigger on the stack and then cast Shock. Because of the way the stack works, the Shock will resolve first dealing two points to its target before you take any damage from Manabarbs. In most cases this will not matter, but if you are at one life and your opponent is at two, this will make all the difference in the world.