The rate of weight gain is how fast your body weight rises during a building phase, usually expressed as a percent of body weight per week. It is the main lever that decides how much fat a bulk brings with it.
Percent instead of kilograms has a reason: half a kilogram a week means something different at 60 kg than at 100 kg. Percentages scale with body weight.
Targets
Iraki et al. 2019 suggest about 0.25 to 0.5 percent per week for beginners and intermediates and around 0.25 percent for advanced athletes. As a warning sign they note that gaining 2 kg a month can be too much for intermediates. A finer split by training years is an interpretation within that range: about 0.4 to 0.5 percent in year one, 0.3 to 0.4 percent in years two and three, and around 0.25 percent after that.
Example: At 80 kg, 0.25 percent per week is 0.2 kg and 0.5 percent is 0.4 kg. This is a worked example, not a target for you.
Why faster is not better
Gaining faster does not mean building muscle faster. In a study of elite athletes (Garthe et al. 2013), fat-free mass rose equally in both groups, but fat mass rose much more in the group eating more calories.
Measure it by the trend
Read your rate of gain from your trend weight, not from two single morning readings. Those swing by a kilogram or more from day to day, and a gain of 0.3 kg a week disappears in that noise. If your rate is clearly above or below target after two to three weeks, adjust your calories in small steps.