We want to switch around the electromagnet’s field every half turn, thereby setting it into continuous rotation.
But how would we engineer such switching?
On the magnet’s rotatable shaft, we add a metal ring that has been split into two slightly separated pieces (in this view, one half ring is to the left, the other to the right).
For obvious reasons, each of these new pieces is called a "split ring."
One end of the electromagnet is wired to one of these split rings (as indicated by the moving red arrow).
The other end of the electromagnet is wired to the other split ring (as indicated by the moving green arrow).