The whole “calories in, calories out” concept isn’t just a concept. You eat in order to provide yourself with the fuel you need to get through the day. If you don’t eat enough, you will struggle to get to the end of the day because you have exhausted your fuel reserves and are running on fumes. If you eat more than you burn up to use as fuel, it gets stored for later use. If you never end up dipping into your fuel stores, they just keep building up. This is why every single health and fitness related magazine, article, website, or blog that you read says the same thing: to lose fat, you need to burn more calories than you consume.
To maintain your current weight, you need to consume only those calories that you actually use in the course of a day.