what is ear plug?

An earplug is a device that is meant to be inserted in the ear canal to protect the user's ears from loud noises or the intrusion of water, foreign bodies, dust or excessive wind.

It is one of those small plastic things.

You put one in each ear to block out noise when you go to bed.

Some earplugs are primarily designed to keep water out of the ear canal, especially during swimming and water sports. 
This type of earplug may be made of wax or moldable silicone which is custom-fitted to the ear canal by the wearer.