You can use the Base64 Decode tool to get the plain text version of your Base64 encoded data. For the reverse operation, please use the Base64 Encode tool.
What is Base64 Encoding?
Base64 is an encoding format used to transform binary data into a sequence of 6-bit ASCII characters (i.e., the Base 64 Alphabet). The standard is presented in RFC 4648 alongside other Base encoding standards like Base16 and Base32. Here is a representation of the Base 64 Alphabet from RFC 4648