Literally all that means is that your body doesn't need as much food to function because you're not growing anymore. Learn to listen to when you're full and when you're hungry - the biggest problem in my opinion is that people grow up with eating habits and they just do them on autopilot. They gain weight because they're used to eating a certain portion or a certain amount of calories. (Also, my freshman 15 can be traced back to college buffet-style dining halls and drinking more than usual).
Compare: my mother eats the same amount and the same kinds of foods she did when she was 20. She's fat. My fiance's mom is skinny, and there are days where she just doesn't eat, or only drinks juice, because she's so in tune with her body. She eats very light, because she feels icky when she doesn't. Doesn't diet to my knowledge, doesn't count calories, but always talking about how certain foods make her body feel.