Clicking joints are caused when boney parts of a joint rub against each other. There may be no pain in such situations but there will certainly be more friction in the joints affected. The cause of the problem may be a thinning of joint fluids or perhaps some trauma in previous months.
Whatever is the root cause of the problem it is certainly true that clicking joints are likely to get worse and result in arthritis eventually. It is therefore important to try and improve the situation before things get worse. In valuable competition horses vets may try injections of hyluronic acid to try and improve quality of joint fluid and general health of synovial membranes. For people who cannot spend a lot of money on such problems supplements can provide a more satisfactory and long term solution.