The two sinkers are two weights on the end of the rope used to fathom the depth. When you chuck the rope in, both disappear until you reach shallow water (off a coast), when the top one stays clear of the water. At this point, you can "see the bottom" between the 2 sinkers. It means that after a long time on the open sea, you've struck land (usually shortly before coming into harbour or a river estuary).

I thought everybody knew this!

:-)