What Base64 actually does
Base64 turns arbitrary binary or text data into a string made up of only 64 safe characters (A–Z, a–z, 0–9, +, /) — the kind of alphabet that survives being pasted into places that were only ever designed for plain text, like email bodies, JSON strings, or old-school data URLs for embedding small images directly in HTML/CSS.
Why encoded text is about a third longer
Base64 packs every 3 bytes of input into 4 output characters, so the encoded result is always roughly 33% larger than the original. That overhead is the trade-off for using an alphabet simple enough to survive any text-only system without corruption.
Worked example
Encoding Hello, World! produces:
SGVsbG8sIFdvcmxkIQ==
The trailing == is padding — it appears when the input length isn't a clean multiple of 3 bytes, so the last group needs to be padded out to a full 4-character block.