A maintenance phase is a period where you eat about as much as you burn, so your weight stays stable. Alongside bulking and cutting, it is the third basic direction.
Eating at maintenance does not mean nothing happens. With progressive training and plenty of protein, beginners and returning lifters in particular often still build muscle and lose a little fat. That is called body recomposition.
When it makes sense
Typical moments are the end of a cut, a break between two building blocks, or a time when you do not want to commit to bulking or cutting yet. A diet break is also a short maintenance phase in the middle of a diet.
After a cut, you raise your calories to your estimated maintenance. In the first few days your weight usually rises a little as glycogen and water fill back up. That is not fat.
Check it with your trend
Instead of a direction, a maintenance phase uses a band around your weight. If your trend stays inside it, your number fits. If it drifts steadily up or down for two to three weeks, you are off your maintenance and adjust in small steps.
Example: At 80 kg trend weight and a corridor of plus or minus 1.5 percent, the band runs from 78.8 to 81.2 kg. This is a worked example. In Lean is law this phase is called "Maintain" and uses exactly this kind of corridor.
Because the scale stays flat, it pays to track your waist, photos and strength numbers as well.