A creatine loading phase is a short period at the start of taking creatine during which you take more than you will later, so that your muscle stores fill up faster. After that you continue with a smaller maintenance amount. The alternative is to take only the maintenance amount from day one, in which case your stores fill more slowly.
What happens on the scale
Creatine pulls water into your muscle cells. The rise starts while the stores are filling and ends when they are full. After that, creatine holds your weight at the new level without pushing it further.
With a loading phase, your stores fill faster, so the step appears earlier and is compressed into a few days. Without one, it is spread over a longer period and is harder to tell apart from real gain in your trend. How big the step is varies from person to person; you are best off measuring it on yourself.
How to read your trend
Do not judge your weekly rate during the loading phase or in the days right after it. Only once the trend runs evenly again does its slope show your real rate. Also leave these weeks out of any maintenance calculation, or you will count water as fat or muscle.
Example: If your trend rises by 0.9 kg in the week you start and then levels off, that much fat would need about 6,900 kcal of surplus. If you did not eat more, it is water. These numbers are a worked example.
Add a note or a marker on day one so you can attribute the step later. If you have kidney disease or take medication regularly, check with your doctor before starting.