This is probably the most commonly asked plumbing question.
In most cases it is caused by the flapper not closing on or making a proper seal around the flush valve. If the flapper is not closing, then jiggle the handle and that will normally fix the issue. If the flapper is worn out and not properly sealing, then you will have to have the flapper replaced. Either way if the flapper does not seal the water will leak into the toilet and the tank will not fill with water to push the float up and turn off the supply of water which can add up to a huge water bill, not to mention wasteful for the environment.
Of course, Tims Plumbing Service can quickly take care of that problem for you.