Base64 is a binary-to-text encoding scheme that represents binary data in an ASCII string format. It's commonly used to encode data that needs to be stored and transferred over media designed to deal with text.
Base64 converts every 3 bytes of binary data into 4 ASCII characters from a set of 64 different characters (A-Z, a-z, 0-9, +, /). This makes the data about 33% larger than the original, but ensures it can be safely transmitted.