What is meant by hashing of a file?

Hashing a file is retrieving its unique hash. Any file is different and they all have other hashes. You can compare hashes with the fingerprint of a human. These hashes serve for many purposes.

Virustotal.com for example, uses hashes to identify files which have already been scanned before. Many distributors of huge files (such as game clients or operating systems such as Ubuntu) often show the MD5 hash of the file. If you hash the file and get the same MD5 hash, the file has been downloaded entirely with no errors.

You can hash a file using various applications. I use HashTab for that.