This is a mail indicator that shows you when you've received new mail.
It mounts onto your mailbox, as shown in the photos (I used hot glue to attach it).

When the mailbox is opened, the indicator pops up to let you know there's mail. To reset it, simply pull the handle.

Use a piece of string or rope to connect the moving parts. Make sure to glue everything in the correct position (on the mailbox).
This step can be a bit tricky, so take your time to get it right.