A weight fluctuation is a change in your body weight from one day to the next that has nothing to do with fat or muscle. As a rule of thumb, daily swings of about one to two kilograms are normal, a bit more for larger bodies and a bit less for smaller ones.
Where the swings come from
Almost always it is water. Carbs are stored as glycogen in muscle and liver, and every gram of glycogen binds about three grams of water. Salt binds water too, so a salty dinner often shows up the next morning. Then there is the content of your digestive tract, which counts on the scale until it has passed. Hard training, stress, poor sleep and, for women, the menstrual cycle also shift your water balance.
Example: The scale shows 1.5 kg more overnight. Using the approximation of roughly 7,700 kcal per kilogram (Hall 2008), that would take a surplus of more than 11,000 kcal in a single day. That does not happen; the explanation is water. This is a worked example.
What it means for you
The daily swing is often bigger than your real change per week. A lean bulk adding a few hundred grams a week gets completely buried in it. If you only weigh once a week, you might compare a high day with a low day and mistake a week of loss for a week of gain.
That is why it is better to weigh daily under the same conditions and judge your trend weight rather than any single reading. More readings mean more noise that cancels out. The useful question is not why you weigh more today than yesterday, but where your trend sits this week compared with last week.