The chirping of birds filled the air with melodies.

Meaning: This means that a particular group of birds were singing and their songs were making the air around them filled with music. It's like saying, The sound of the birds singing was so loud that the air was filled with their beautiful songs.

Choice of Words: The is used to specify that we are talking about a particular group of birds. Chirping is the sound that birds make. Of is used to show that the chirping belongs to the birds. Filled means to make something full. Air is the invisible gas that surrounds the Earth. Melodies are pleasant sounds that are created by music. When you put them together, "The chirping of birds filled the air with melodies" means that a specific group of birds were making a lot of noise and the sound was all around.