Subtitles Hash/Name

[CheapTom]

Hi,
Just a general question about the differences between Search by Hash and Search by Name? Searching for the name of a file, is a gimme, but even after researching this, I still don't know what a hash is and how subtitles are found by Search by Hash.
Is there an article that explains this simply, or can someone help? I'm just curious, and this question isn't related to any problem I'm having, thanks