A wave is a disturbance that transforms energy through matter or space, with little or no associated mass transport. Waves consist of oscillations or viberations of a physical medium or a field around relatively fixed locations.
Now this is a definetion from wikipdia, lets try to understand it with a example.
How do you think sound travell from one person or from oneplace to another??
well it is through mechanical waves, when we speak, a viberation is occured in the air molecules and this vibration travell to another persons ear, but some of the energy is lost, therefore when a person speak from far distance you can hear a samll intensity of sound.