The laughter filled the air of the afternoon.

Meaning: This sentence is saying that the sound of people laughing could be heard all around during the afternoon. It's like saying, The afternoon was full of the sound of people laughing.

Choice of Words: The is a determiner that refers to a specific noun. Laughter is a noun that means the act of laughing. Filled is a verb that means to make something full or to occupy a space. Air is a noun that means the invisible gases that surround the Earth. Of is a preposition that means belonging to or connected with. Afternoon is a noun that means the time of day between noon and sunset. When you put these words together, you get a sentence that describes how the sound of laughter was everywhere in the air during the afternoon.