1) The flapper is leaking, causing a constant refill. (The water level in the tank would be dropping, and then refilling) if that happens, either the large nut on the bottom of the tank needs to be tightened, or the flapper replaced.
2) The fill valve does not shut off. (The water would be going over the overflow tube) If that happens, the fill valve needs repair or replacement.
2) The fill valve does not shut off. (The water would be going over the overflow tube) If that happens, the fill valve needs repair or replacement.