When something subsides, it goes down or becomes less strong.
If something subsides, it is usually a good thing because it means the bad thing is getting better.
Examples:
After the storm, the wind subsides.
The pain in my head subsides when I take medicine.
The water subsides after the flood.
The noise of the cars subsides when the traffic light turns red.
The anger in his voice subsides as he calms down.
The waves subside and the sea becomes calm.