A water change is the routine removal of part of the tank water and its replacement with fresh, dechlorinated water — the main way you export nitrate and refresh minerals.
Regular partial changes (a common starting point is around a quarter to a third of the tank weekly, adjusted to your stocking and nitrate readings) are the backbone of a healthy tank. Match the new water's temperature and treat it with dechlorinator.
You almost never need to change 100% of the water or clean the filter and gravel at the same time — that strips the beneficial bacteria and can trigger a mini-cycle.