About maintenance level
Your maintenance level is your current body weight and the amount of calories you need to eat to hold that weight steady.
Why know your maintenance level?
Knowing your maintenance level enables you to drop or add calories from it to either add or lose weight in a precise and controlled manner.
I don't know my maintenance level?
The best way to know is through trial and error until you find out. Do this by sticking to the average recommended calorie intake for men and women (2500 and 2000) and adjust accordingly from there to lose or gain weight. Once you stop losing / gaining weight then you know. Everybody is different and will have their own levels, overtime you will know yours.
I was losing / gaining weight from maintenance but now it has stopped?
This is because your body is now at a new weight that is equal to the calories you were consuming to get to that weight. If you keep eating the same calories you will stay at this weight. You now need to adjust your calorie intake from this new maintenance to drop or gain more weight.
Balloon example
The above image shows a balloon after 5 pumps of air. Once a month you minus one pump of air into the balloon. Now the balloon contains 4 pumps of air and is smaller. One month later it is down to 3 pumps and you stop there. Every month from here you pump only 3 pumps of air. Ten months later the balloon is still the same size at 3 pumps.
Similar to calories, your body will size down until it stops at that level of size for those calories.