PDCCH order in 5G NR is a way of network instructing the UE to trigger a Random access Procedure. Typically RACH is triggered by UE and there are a number of reasons why RACH can be triggered by UE. But the network can force the UE to trigger RACH when it detects that UE is out-of-sync in Downlink. So in that scenario, the network triggers a PDCCH order by sending a DCI Format 1_0 on the SSB beam index UE is camped along with a PRACH preamble and RACH occasion.