This is the traditional bugle call known as "To Arms", and it is used by military forces to order troops to assemble with their weapons. As such it serves a broadly similar purpose to calls such as Assembly and Fall In. See the links at the left to download the sheet music or audio files. More bugle calls are listed in our article about the history and cultural impact of Bugle Calls
The image below is the first page of the sheet music, and here you can download the full sheet music for To Arms (bugle call) in PDF format.