A Marlin Spike is a simple tool (most commonly steel, but I made this one out of an oak dowel) that is tapered to a narrow but rounded point on the end. It is often curved but not always. It is used to help organize the cords and clear a path while tying a complicated knot. You can see demonstrations of its use in the various flogger tutorials. It is almost impossible to tie Double Diamond Knots, Footrope Knots, Manrope Knots, etc., without one.
I made this simple Marlin Spike out of the small piece of dowel I happens to have around the shop. I used a drill as a rudimentary lathe. I clamped one end the dowel in the chuck of the drill, turned on the drill and slightly rounded the other end using a piece of sandpaper. Then I clamped the slightly rounded end in chuck, turned on the drill and, using a piece of sandpaper, made the tapered point.
Note that the point is not super sharp. You want it narrow enough to be able to work between the strands of a knot but not so sharp that it can pick your rope apart