A melody in music is a group of notes of various pitches (how high or low a note sounds) which are played one after another. Together they make a tune in the same way that a group of words make a sentence. Some people like to sing melodies. Melodies have rhythm (the length of the notes).
Some music has more than one melody happening at the same time. When this happens throughout the whole piece, it is called polyphonic music. Rounds and fugues are types of polyphonic music. If the other melody only happens sometimes, then the second melody is called a countermelody.
The adjective of melody is melodic. There is also the adjective melodious which means: 'a sweet-sounding melody'.
Music that does not have an obvious melody may be hard to understand (unless it is focused on rhythm, such as a drum solo).