A weight loss plateau is a stretch where your weight stops falling for a while even though you are dieting. The key is telling a stalled scale apart from a stalled trend.
Apparent plateau
Even with a perfectly kept deficit, your trend can move sideways for two to three weeks because retained water masks fat loss. Common triggers are a new training program, starting creatine, salty food or, for women, the menstrual cycle. Fat loss keeps going during that time, and the weight often seems to drop suddenly afterwards. If you cut calories in such a week, you are correcting water.
Real plateau
As your weight drops, so does your energy expenditure: a lighter body needs less energy, and many people unconsciously move less during the day. Trexler et al. 2014 describe this adaptation. Your deficit gets smaller over the weeks even though you eat the same. At some point it is no longer enough for visible progress.
What to do
First check whether your trend has really stalled for two to three weeks, not just the scale. Also check whether you ate consistently and weighed in without gaps during that time. Only then convert the gap into calories.
Example: Using the approximation of 7,700 kcal per kilogram, every tenth of a kilogram per week that you are behind plan equals about 110 kcal a day. If you are 0.3 kg a week short, that is about 330 kcal. This is a worked example.
Then wait another two to three weeks. On long diets, a diet break can also help, mainly with hunger.