Encode binary information 8 bits into ASCII. 3 x 8 bits binary are concatenated to form a 24bits word that is split in 4 x 6bits each being translating into an ascii value using a character ordered in following list :
ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/ 0000000000111111111122222222223333333333444444444455555555556666 0123456789012345678901234567890123456789012345678901234567890123
[what makes 26 * 2 + 10 + 2 = 64 values]
Since it encode by group of 3 bytes, when last group of 3 bytes miss one byte then = is used, when it miss 2 bytes then == is used for padding.
Base64 itself does not impose a line split, but openssl uses it in PEM context hence enforce that base64 content is splitted by lines with a maximum of 80 characters.