As a sound is propagated away from its source a certain proportion of it reaches the listener as direct sound, following a straight path from source to ear. A large proportion of the sound, however, radiates in all directions from the source and encounters the boundaries (walls and objects) of any enclosure the sound occurs within. 
If these boundaries have reflective surfaces some of the sound bounces back into the room and is heard as delayed and coloured sound.  The amount of sonic absorption these surfaces have determines the amount or density of reflection and how long the reflections persist or decay.  The quality of the material the surfaces are made of also determines what frequencies it will tend to more readily absorb or reflect resulting in characteristic frequency colouration of the reflected sound. 
These cues:  delay time;

frequency colouration;

reflection density 
or reverberation; 
are interpreted cognitively to give us a perception of the space in which a sound occurs. 
direct sound early reflections reverberation