ScrLK= Scroll lock. It means that the arrow keys move the page up and down instead of the mouse. It is toggled just like Number lock and Caps lock. You have to press it again to turn it off.
SysRq= System Request. It allows you to perform a few low level commands- doesn’t matter what state your computer is in. It recovers your computer when frozen and can reboot the computer without filesystem corruption.

Edit:I don’t know how useful they are, I don’t have a computer that has them. The SysRq one seems pretty useful though.

If I remember correctly, the SysRq is left over from pre-internet days when communications to the host was done using X.25 or even more primitive protocols. I believe that IBM’s old PROFS management system, which was green characters on a CRT (not a GUI) had you press SysRq to ‘wake up’ the machine and tell it you were about to interact with the mainframe.

It also played some role in switching from one operating system (mainframe) to another on old PCs. (Hence the name SysRq)

Now (2017) it has been coopted, or adapted by Linux to mean something totally different. But historically, to old IBM PC users, it had real use.