Getting rid of or reducing an echo in a room involves limiting the number of solid surfaces off of which sound waves are able to bounce. This includes installing carpet to rid a room of hard floors or hanging up curtains to prevent as much sound as possible from bouncing off walls.
In addition to carpets and curtains, acoustic tiles help reduce echo. These tiles are designed to absorb sound waves. Theaters often cover their walls and ceilings in acoustic tiles to prevent echoing during movies. Placing furniture around a room creates less empty space for sound to bounce around in. Although an echo may still present after these steps, it isn't as noticeable. If the echo is being caused by speakers, echo-reducing speakers often eliminate the problem.
An echo occurs when sound waves bounce off of a solid surface and return to the source of the sound. Since the sound has to travel to a hard surface and back to the source, it creates a repeating effect. If the room is large enough, and if there are multiple hard surfaces, the sound can be repeated many times. The further the sound has to travel from the hard surface and back to the source, the weaker the sound becomes.