Water retention means your body temporarily holds more water than usual, so the scale goes up without any change in your fat. It is the most common explanation for jumps that appear overnight or within a few days.
Typical triggers
Glycogen is the biggest factor. Your body stores carbs in muscle and liver together with about three to four parts water per part glycogen (Kreitzman et al. 1992). After hard or unfamiliar training, extra fluid also collects in the stressed muscle, and soreness is the sign of it. Salty meals are another commonly cited trigger, although the research on salt is less clear-cut. Add to that starting creatine and, for many women, the menstrual cycle.
Spike or step
How water retention shows up in your trend weight depends on whether the trigger is short-lived or lasting. A pasta night or a hard leg session creates a brief spike that fades after a few days. If you eat more carbs for good or start creatine, the trend takes a step up and then carries on at its old pace. Fat, by contrast, shows up as a slope that continues week after week.
During a cut, water can mask fat loss for several days, after which your weight seems to drop all at once. Cutting calories during that time means correcting for water. So log the high readings too, and only judge the slope of your trend over two to three weeks. If you notice swelling or a gain you cannot explain, take the question to a doctor.