Rule of split horizon says,

If router get some routing update from it's neighbor, it will send it to all the routing enabled ports except the one from the update came in on.

In Figure, When R2 receive an update about 10.0.0.0/8 network, it will never send it back to the R1 on same port.

In this example, R2 will send an update to R3 & R4 respectively.

The Split Horizon concept is used to reduce extra traffic overhead which leads to high CPU & Bandwidth utilization.

I hope this explanation is satisfactory.