F5 is a key on most keyboards, found in the function key row at the top (F1 through F12). It is the leftmost key of the middle third, and is right above either the number 5 or 6, depending on the layout of your keyboard at home. It does no typing, because it is a function key.
F5's most common purpose is the refresh key in browsers, and this is what it is known for and immortalized as. Many common internet activities benefit greatly from having a quick-access key to refresh the browser page, be it in the attempt to get a milestone pageview or to view the resultant changes of making an edit to some file or other that was used in the generation of the current webpage. Some browsers refresh by simply reloading the state of the page as it was downloaded from the server: a force refresh, Shift + F5, will usually set these guys straight.
F5 is the refresh key in many other settings, too. The most prominent example is the Windows Explorer application we all know and love: it will refresh the current folder display upon the pressing of F5. On a Mac, it shifts focus to the toolbar or something, so that's kinda refreshing, I guess? It's hard to compile a list of everything that it does because it varies so much from system to system and from program to program. Read the readmes, kids.
F5 as a key for refreshing is also the inspiration for an album by 'Слот' (literally 'slot', usually rendered 'SLoT' when in English), a Russian alt/nu metal band, as well as the album's leading track. F5 is their most recent work as of 2012, and it's a little less metalhead metal than it was in Break The Code, but it's still as energetic and eloquent as ever. Personally, my favourite tracks are «Манифест»/"Manifest" and «Одинокие люди»/"Lonely People", but all of it finds a spot in my "love this stuff" playlist.
It has probably inspired a great deal more pop culture, especially with the rise in popularity of geekdom recently, but again, this sort of thing is difficult to google for: you just sort of come across it and smile or nod knowingly.
F5 will always have a special place in our hearts and minds: the brave little key we turn to whenever we need to take another look at something. Never honoured, never noticed, but always needed and always unselfish. A true workhorse.