Yoyo dieting is when a person goes through repeated cycles of losing weight and regaining it, usually in short time frames too.
This is sometimes referred to as weight cycling and can have harmful effects on both your body and mental health.
Yoyo dieting can result in your metabolism slowing down, making it even more difficult to lose that weight again.
Further red flags of yoyo dieting include muscle loss, as each time you go through that cycle of losing and regaining, you lose both muscle and fat, but when you regain the weight, most of it is just fat.
The best way to lose weight healthily and sustainably is by honing in on the basics - eating a balanced, nutrient-dense diet, exercising regularly with activities you enjoy and going easy on all the other unfavourable foods/drinks etc.