Base64 normalization is a process that transforms the Base64 string to its canonical form. For example, it discards invalid characters, removes unnecessary whitespaces, adds padding characters, or replaces index characters according to the chosen standard. Thus, the Base64 normalization tool also allows you to convert online one Base64 standard to another.
Please note that even if the normalization tool discards invalid characters, it does not repair malformed Base64 strings. Therefore, if you have a “undecodable” string which cannot be decoded by other tools, try Repair Base64 and make it decodable.
By the way, you may be interested in other online tools such as standard detector, validator, or character encoding. They might be useful if you want to get more information about your Base64 string.