PC keyboards have always had Function Keys, just as mainframe terminals did before them – thank IBM for cementing F-keys on the modern keyboard, though. Even Apple Macs had function keys, though the latest fad is to replace them with a Touch Bar – among other things that were replaced.
Some terminal keyboards had up to 24 function keys, with the idea that different application would have various commands assigned to each. The modern multi-tasking, graphically-oriented operating system has largely done away with the need for function keys, but certain commands persist and are supported widely – ALT-F4, for example, will pretty much always close a Windows application. CTRL-F4 will mostly close a window or tab.
F1 usually means “help”. F2 tends to rename the thing you’ve selected. F3 normally does a “search”. F5 usually refreshes whatever you’re viewing. There’s more.
If you’ve a Surface Book, check out Paul Thurrott’s commentary from a while back, and if you’ve any other Surface device you might find the doubling up of function keys and other regular keys causes grief at times, as having the toggled “Fn” key locked on (so as to use the F-key functions) will nullify the other functions printed on the same keys. Losing access to the key that mutes your speaker or presses play/pause might be a minor annoyance, but forfeiting the Home, End and PgUp/PgDn keys can be a right pain if you’re editing text or moving around a spreadsheet. There’s no easy way of avoiding this, other than just being aware of whether you have the Fn key toggled or not.
Somewhat obtusely, Surface Book/Pro fans may not realise that the Fn key doesn’t just toggle on and off, but can be used in conjunction with other keys to provide spot functionality – the most useful being the Fn+Del and Fn+Backspace key combinations, which change the screen brightness up and down. Certainly more regularly useful than the keyboard brightness settings that share the F1 and F2 keys. This nugget was found in the Surface Book user guide, published along with guides for other Surface devices, here.
One of the best hidden function key combos to remember, though, is the F4 key within Office applications – it repeats the last thing you did, from colouring some text to lots of other stuff. If you’re applying formatting, for example, rather than using the Format Painter command in Office apps, you could simply set the format on one paragraph/cell/whatever, then select another one to apply the same formatting just by pressing F4, and you can continue to apply the same settings by selecting some more/pressing F4, etc. Magic.