Encode or decode text to/from Base64 format, run entirely on this server with nothing kept afterward. Comes up often with vmess:// links specifically -- they are a Base64-encoded JSON document, and decoding one is how you read the address, port and path a link actually contains.
Base64 encodes arbitrary text into a compact ASCII form. It comes up constantly in tunnelling because vmess:// links are a base64-encoded JSON document -- decoding one is how you read the address, port, id and path a link actually contains.