Base64 is a way to represent files as text. By using text instead of the original file it can be easier to place it on a webpage or to share it with other people. For example, HTML tags accept base64 data in their src fields. The base64 text contains the contents of the file but anything else such as the filename will be lost. The size of base64 output is generally a lot larger than the size of the original file. It makes sense to only use base64 text when you really need it.
When you use these converters the file or text you use will stay in your browser; the file or text is not sent anywhere. Note that if you try to convert a very large file, your browser may hang because your device cannot handle converting a file that large.
The tool for converting base64 text to plain text will only work if the plain text is also regular text. For example, it won't work with binary data.
This tool is provided as-is without any guarantees, regarding security or otherwise.
Don't paste input that you don't trust! There may be safety issues.