salary sacrificing

[Traveller]

Can anyone shed a light on what this is please? I've been told I'm eligible, and I'll be working for a public hospital, but I don't really understand it (particularly things like how I can offset my mortgage against my income). I'll get more information from HR in a few weeks but I suspect this will come in the form of a large document with lots of words so if anyone can sum it up for me I would appreciate that