A permalink is just a link to the anchor point where a specific post is located.
To use it, just right-click on the permalink link on the post you wish to reference, choose "copy link" and paste it wherever you wish to reference the original.
i.e. instead of providing people with a link to the thread (as shown at the top of your browser window), you are instead providing them to a link to the specific post you wish to highlight.... not much use on short threads like this one, but on larger hamster wheel ones like MH370, it can be useful to point people to the specific post so they don't have to trawl through thousands of pages.