A grub screw is a type of screw with no ‘head’ – rather than having a dome shaped top, they are flat and tend to have a hexagonal shape indented into them so that they can be turned by an allen key.
Removing the retaining screws will allow you access to the inside of your boiler pump.
When removing these screws, you will only need to use one size of bit. The screws do not come in any other sizes other than 4mm (5/32″), 5mm (13/64″) or 6mm (15/64″), so trial and error will very quickly let you know which is the right bit for the job!