3GPP release 15 defines a four step RACH procedure.
And in 3GPP release 16 it is called 2-step RACH Procedure.
Follows a good reading to understand the concept:
http://www.dheerajsharma.net/3gpp-rel-16-2-step-rach-procedure/
The main goal here is to reduce Latency and also control-signaling overhead.
Now there’s a single round trip cycle between the UE and the Nodes.