Youtube2mp3 converts the YouTube video and then creates links for downloading the mp3 file.
It creates 2 links for downloading the mp3 file: a direct link to the file and a link to the file in zip format.
The zipped file is simply a compressed file. To open it, you need a zip extraction program.
The most common software for extracting zip files is Winzip.
New versions of Windows, however, feature integrated software for extracting and reading the content of zip files.
In other words, if your operating system cannot read zip files, you need to install decompression software like WinZip.