Stagnating means not moving or changing. It can also mean becoming foul or rotten from lack of movement.
Stagnating has a negative feeling. It usually means something is not good or healthy.
Examples:
The water is stagnating in the pond.
The economy is stagnating, which means it's not growing.
The air in the room is stagnating.
The old bread is stagnating in the cupboard.
The company's profits are stagnating, which means they're not getting bigger.
The pond is stagnating and it smells really bad!