Open the top of the tank and watch what happens while you flush. There are any number of things in there that can make the toilet ‘run’.
When you flush, the contents of the toilet bowl go down the drain, and a drain is opened in the tank so that the fresh water in the tank can go into the bowl. When all the fresh water has drained into the bowl, the drain closes and the tank starts to refill with fresh water. That’s the hissing sound you hear for a minute or so after you flush.
When the water level gets high enough, there’s a float in there that gets pushed up by the top of the water. The drain closes and the fresh water stops refilling the tank.
If your toilet is ‘running’, something is stopping the drain from closing, so the water that comes into the tank just goes right back out into the bowl and keeps going on through, so you waste a lot of fresh water. The seal around the tank drain might be bad, or the chain that connects the flush handle to the drain might have gotten tangled or stuck under the seal. Unless it’s a faulty part like a bad seal, jiggling the handle or lifting the hood and manually straightening things out (which is safe because the water in the tank is as clean as the water from your sink faucet) does the trick.