The IC (Integrated Circuit) Socket is a footprint where we can firmly connect any IC (chip) to our board. We solder the socket into our board because we don't want to solder the IC itself -- it can be fragile and easily damaged by the heat from our soldering iron. We won't insert the 555 chip into this socket at the very end of assembly, so that it is never exposed to the strong heat of our iron.
Make sure to line up the notch on the socket with the notch on the white outline (silkscreen) on the board. Fit in, making sure all 8 pins are through the board and solder the socket in place.