If something is stagnant, it is not moving or flowing. It is still and not changing.
Stagnant can make people think of dirty water, lack of progress, or being stuck.
Examples:
The water in the pond is stagnant.
The air in the room feels stagnant.
The stagnant economy is not growing.
The stagnant river is not good for fishing.
In the stagnant air, it's hard to breathe.
The stagnant water is full of bugs and smells bad.