Water weight is the part of your body weight that comes from a change in stored water and can come and go within a few days. It is neither fat nor new muscle, but it weighs just as much on the scale.
The main source is glycogen. Every gram of glycogen is stored in muscle with at least three grams of water (Fernández-Elías et al. 2015). When you change your carb intake, your water weight changes with it. Creatine also pulls water into muscle cells.
Example: If your glycogen changes by 300 g, a ratio of 1 : 3 to 1 : 4 adds roughly 900 to 1,200 g of water. Together that is about 1.2 to 1.5 kg on the scale. As fat, the same amount would take a surplus of roughly 9,000 to 11,500 kcal. These numbers are a worked example, not a prediction for you.
Where it fools you
When you start a diet with fewer carbs, part of the fast loss in the first week is glycogen plus its water. If you extrapolate from that week, you overestimate your pace. Switching to a bulk or starting creatine does the same in the other direction, and if you cut calories in alarm, you take away the surplus you just planned.
In your trend weight, water weight shows up as a step that forms within a few days and then stays put. So only measure your weekly rate once the step has settled. A marker on the day of the change helps you recognize the step later.