Unix-to-Unix encode is a utility to encode binary data into ASCII text characters. It is used to encode binary data, allowing it to be exchanged through the uucp mail system.
Unix-to-Unix encode is also known as Uuencode.
Techopedia explains Unix-to-Unix Encode
Uuencode is a program used for transforming a file into a form allowing transmission via email. An example of such file is a Word document, which cannot be sent via email in its original form -- but rather can be sent only in encoded form. Uudecode would then transform the encoded file into its original form.