Definition: The ZIP file format is a data compression and archive format. A ZIP file contains one or more files that have been compressed, to reduce file size, or stored as is. They are saved in .zip format.
This definition uses material from the Wikipedia article "ZIP", which is released under the Creative Commons Attribution-Share-Alike License 3.0.
To unzip a file requires an "Unzip Program". If your operating system does not have one, you will have to find one.
Unzipping a file will decompress the ZIP file to reveal the individual files. The method is similar between operating systems but may differ slightly in the details. The basic process is to select where to save the ZIP file on your computer and then decompress it.