A binary-to-text encoding is encoding of data in plain text. More precisely, it is an encoding of binary data in a sequence of characters. These encodings are necessary for transmission of data when the channel does not allow binary data (such as email or NNTP) or is not 8-bit clean. PGP documentation (RFC 2440) uses the term ASCII armor for binary-to-text encoding when referring to Radix-64.
Read more about Binary-to-text Encoding: Description, Encoding Plain Text, Encoding Standards