Water is running into the toilet bowl all the time.
The valve that lets water into the cistern is old or damaged and needs replacing.
What you can do
- Not recommended for doing it yourself.
You should fix this for three reasons: 1) it wastes water, and therefore costs you, 2) it can stain the bowl, making it difficult to clean, 3) a leaking valve can spray water out of the cistern and cause water damage to ceilings and walls.
This is a job for a plumber or an experienced do-it-yourselfer with a good range of tools. If that’s you, there is a sequence of picture instructions in this pdf you can download: Replacing a toilet cistern Torbeck valve.pdf (To download, right-click and ‘Save As…’)