A melody that stays on the same pitch gets boring pretty quickly. As the melody progresses, the pitches may go up or down slowly or quickly. One can picture a line that goes up steeply when the melody suddenly jumps to a much higher note, or that goes down slowly when the melody gently falls. Such a line gives the contour or shape of the melodic line. You can often get a good idea of the shape of this line by looking at the melody as it is written on the staff, but you can also hear it as you listen to the music.
You can also describe the shape of a melody verbally. For example, you can speak of a "rising melody" or of an "arch-shaped" phrase. Please see The Shape of a Melody for children's activities covering melodic contour.