Question

What do you mean by decomposers? And how are they useful to us?

Answer 1

plural noun: decomposers

an organism, especially a soil bacterium, fungus, or invertebrate, that decomposes organic material.

a device or installation that is used to break down a chemical substance.

Answer 2

decomposer are the organism that decomposes the organic material such as the remains of dead organisms.
There uses-
because of decomposer plants get proper nutrients.
They help in release of nitrogen from the dead organisms