When sound is produced in a room or other enclosed space echoes build up. This is called reverberation. When the sound source is removed, the echoing sound is absorbed by the surroundings and the sound level decays.
The reverberation time (or RT60) describes the time it takes for the sound to decay by 60 dB after the sound source is removed.
The reverb time is affected by the size and shape of the room, the building materials and techniques used and all objects (including people) within the room. Long reverberation times can make speech unintelligible, short reverberation times can make music sound "dead". The ideal reverb time depends on the size and intended use of the space.
Before measuring the reverberation time a source of noise must be introduced, and then removed. A pink noise source is usually used for this purpose.
As defined above, the RT60 is the time taken for the sound to decay by 60 dB when the source is removed. In a real environment this can be difficult to achieve, so it is more common to use the RT30 or RT20. These are based on the time taken for the sound to decay by either 30 or 20 dB respectively, and this is then extrapolated to 60 dB.
When measuring the RT30 or RT20 the sound level meter should ignore the first 5 dB of decay. As you should also avoid measuring any lower than 10 to 15 dB above the background noise, it means that your noise source should be at least 45 dB above background for RT30 and 35 dB above background for RT20. It is for this reason that RT60 is difficult to measure directly, as the background noise would have to be low and the noise source exceptionally high.