Tape

Holodisk on which you can upload to your pip-boy or write your own message.

Where to find one?

You can buy them at Boneyard.

What to do with it?

If you want to upload holodisk message to your own Pip-boy select "Hand" icon from inventory menu. You can read message in your Pip-boy archives. Use on it "Science" from inventory level to write your own message.