What are mangroves? what is the importance of mangroves? why are they decreasing?

Mangroves are *massively* important and we hack them down at our peril. They regulate weather, can protect against severe weather and flooding, they prevent soil erosion, they are home to many species that simply don't live elsewhere (including plants with extracts that include life-saving properties, they are some of the most biodiverse and productive places on the earth.

If you want an example, half a million people died in a severe weather event in the 1970s. It simply wouldn't have happened if the mangroves hadn't been cut down in Bangladesh.