This tool enables you to turn text and images into an ASCII string format via translation to a Base64 string. This is for example used in emails to transfer attachments. The term Base64 is coming from a specific MIME content transfer encoding. This MIME implementation uses A-Z, a-z and 0-9 to encode the data.
As an Example you can take the text "Hello World!" Base64 encoded it would turn to "SGVsbG8gV29ybGQh" and if you decode this string again you will get your "Hello World!" back. Further details you can find on Wikipedia.