A more thorough explanation:

Definition: A drop letter is a letter that is addressed to someone in the delivery area of the post office where the letter was posted.

Example: If you live in New York City and you want to send a letter to your friend who also lives in New York City, you can write their address on the envelope and drop it in a mailbox. This is a drop letter because it is being delivered within the same delivery area as the post office where you dropped it off.

Explanation: A drop letter is a type of mail that is delivered within the same delivery area as the post office where it was posted. This means that the letter does not need to be sorted and sent to another post office for delivery. The example illustrates this by showing how a letter can be sent to someone in the same city without needing to go through additional processing.