Simply, Base64 is a form of information encoding that allows 8-bit binary data (from 00000000 to 11111111 which cannot entirely be typed on a keyboard) to be converted into a form that can. The upshot is that binary data can then be fed into systems that normally couldn't accept it. This is how attachments, like photographs, can be injected into text based systems like email. Most times you don't even need to know about this, but in case you do...

This trinket will return your B64 encoded block of text (called a "string" in computerese). It is limited limited to 64KB.

Note. Base64 is NOT encryption as the original characters can be very easily decoded.