Closed terrariums come in many shapes and sizes, but once it's environment has been stabilized, there is little to no maintenance in the future.
A closed terrarium will have its own ecosystem. Water in the bottom of the terrarium will be taken up by the root system. This will transpire out of the leaves during photosynthesis, and create condensation on the sides. This condensation will "rain down" back into the soil and start the cycle over. This leaves little to no maintenance to you once the cycle is set. How do you normalize and regulate this cycle? It's important to make sure the light and the water levels are suitable for plant growth.
Lighting: Too bright of direct light on the glass will create excess condensation which can lead to mold and plant rot. With a closed terrarium, lower light levels are safer. You will want to give low indirect light or direct light for a max of about 3 hours a day. You should see condensation in the morning or afternoon, but not all day long. If you do see this, feel free to remove the lid for a few hours to let some of that moisture evaporate. Try again, and adjust the next day if needed.
Watering: Initially, water will be placed inside the closed terrarium. Usually, this is filled within the rock/drainage layer and not into the soil. Too much water and the container can be constantly fogged with condensation, and mold can grow inside the vessel. Not enough water and there won't be any condensation, and the water cycle will be broken. Too much water? Open the lid and let some evaporate. Not enough, it's simple, just add some!
This quick cheat sheet is meant to give you tips and tricks for how to best care for your plant. All of the information above is based on my personal experience and advice with houseplants.