Antibiotics kill bacteria that make you sick. But they also kill "good bacteria" -- like the ones that are found normally in your intestines. When these bacterial are killed by an antibiotic one consequence can be diarrhea. One of the good bacteria killed is known as Acidophilus.
Acidophilus bacteria are found in some milk products like sour milk or yogurt (if they weren't killed prior to putting the yogurt in the carton.) Another, more reliable, source are Acidophilus capsules which can be found by the bottle in the section of the grocery store or pharmacy where vitamins and minerals are found.
I don't take the Acidophilus capsule at the same time as the antibiotic, to avoid killing them. But I do take at least one capsule each day that I take an antibiotic.