Let's start by explaining what total dissolved gas (TDG) is. Basically, it's just air mixing with water. Technically, the air is dissolving into the water. Because you can't see the air, you don't see this mixing process. To create an image in your head, think about pouring sugar into water. Some of it mixes and dissolves into the water, and some drops to the bottom. How much becomes part of the water depends on different things you can do, like stirring the sugar in.

When something causes air to dissolve into water, the water's nitrogen and oxygen content (which are the main properties of air) can go way up. This is called supersaturation.