Probiotics are good bacteria that live in the GI tract. They sell them for people and bunnies. In the US, we can usually find them at the big pet stores, as a product called Bene-Bac, or at farm supply stores as a product called ProBios. In Canada, it seems to be harder to find them. You can look at the pet store but another thing that will work is to go to the human pharmacy or health food store and buy acidophilus capsules. They'll be little pills you can cut open and sprinkle the contents onto the food. Don't give any milk products that contain acidophilus--the capsules are the best option.
The rabbit GI tract has a really delicate balance of good and bad bacteria. One whole section of the GI tract, the cecum, is where these bacteria ferment normally undigestible things, like hay, into things that the rabbit's body can process and gain nutrients from. The bacteria process these things, and the final product is called cecals which are eaten by the rabbit. Normally they look like a blackberry or a small cluster of grapes. Usually we don't see our bunnies eating them or see them left on the floor because they eat them right when they're made. If a bunny doesn't eat them, or they are abnormal in shape or texture, something may be wrong. A different shape or texture usually means that the bacteria in the cecum aren't normal.
Abnormal bacteria in the cecum can be pretty serious, but probiotics and a diet that is low in carbohydrates/sugars, and limited in protein should clear it up in a few days. The bad bacteria in the cecum can overgrow the good so that they are the predominant population there. The watery stool can lead to dehydration, plus the rabbit's not getting the right nutrients because they're not eating it. The bacteria themselves can be pretty dangerous if they grow out of control too, poisoning the rabbit's blood. The bad bacteria thrive on carbohydrates and sugars. I have had bunnies get problems like this after eating too many treats.