Creatine is a substance your body makes itself; it is stored in muscle and helps supply energy during short, intense efforts. As a supplement, usually creatine monohydrate, it tops up those stores. It is the best-studied supplement in strength training: according to the International Society of Sports Nutrition position stand (Kreider et al. 2017), it can improve performance in short, intense efforts.
Why the scale goes up
Creatine pulls water into your muscle cells. That water has weight, but it sits in the muscle, not in fat tissue. Your trend weight therefore steps up after you start and then runs flat again. Fat gain looks different: it needs a sustained surplus, and the trend keeps rising week after week. How big the step is varies from person to person. If you stop taking creatine, the trend comes back down.
Example: If your trend climbs by 0.9 kg after you start and then levels off, that much fat would need roughly 6,900 kcal of surplus, using the approximation of 7,700 kcal per kilogram. If you have not been eating more than before, water is the better explanation. These numbers are a worked example, not an expectation.
What to do in practice
Only judge your weekly rate after the step. In a cut, the step can mask fat loss for a few weeks; a shrinking waist then shows that you are still making progress. Add a note or, in Lean is law, a marker on the day you start, so you can attribute the rise later.
For healthy people, creatine is considered safe and well tolerated. If you have kidney disease or take medication regularly, talk to your doctor first.