Direct answer
Base64 converts bytes into printable characters so binary data can travel through text-oriented systems. Standard Base64 uses uppercase letters, lowercase letters, digits, plus, slash, and optional equals-sign padding.
Encoding is not secrecy
Base64 has no secret key. Anyone who recognizes the format can decode it. That makes it useful for representation, transport, and puzzles, but unsuitable as a confidentiality mechanism.
How to recognize it
Short Base64 strings often use a limited alphabet and lengths related to four-character groups. Context matters because ordinary identifiers can look similar.
Multi-stage puzzles
A Base64 string can itself be hidden inside HTML, metadata, or another carrier. In that case, extraction and decoding are separate steps.
Frequently asked questions
Is Base64 secure?
No. It is encoding, not encryption.
Why does Base64 sometimes end with equals signs?
Padding can be used so the encoded output aligns to complete four-character groups.