PDCCH Order is a mechanism by which eNB force UE to initiate PRACH. As you know, one of the most important purpose of RACH is to let UE obtain the synchronization with network and establish the iniitial connection pipe. In most case, the decision to initiate (trigger) PRACH is done by UE side. But there is some case where Network (eNB) need to force UE to initiate the RACH. PDCCH Order is the mechanism. The role of PDCCH Order in normal LTE operation can be illustrated as shown below. (I think the overall description of PDCCH order is much better described in PDCCH Order in LTE at Simpletechpost).
You would find statement of PDCCH order in 3GPP as shown below.
36.212 - 18.104.22.168.3 says
DCI format 1A is used for the compact scheduling of one PDSCH codeword in one cell and random access procedure initiated by a PDCCH order
36.212 - 22.214.171.124.12 says
DCI format 6-1A is used for the compact scheduling of one PDSCH codeword in one cell and random access procedure
initiated by a PDCCH order.
36.321 - 5.1.1 says
The Random Access procedure is initiated by a PDCCH order, by the MAC sublayer itself or by the RRC sublayer. Random Access procedure on an SCell shall only be initiated by a PDCCH order.
As you saw in the sequence diagram shown above, PDCCH Order is triggered by DCI 1A. Actually, the physical entity of PDCCH Order is DCI 1A. In other words, PDCCH Order is a special form of DCI 1A with the fields as described below (NOTE : 36.212 - 126.96.36.199.3 says "Format 1A is used for random access procedure initiated by a PDCCH order only if format 1A CRC is scrambled with C-RNTI and all the remaining fields are set as follows:") .
< 36.321 - Table 7.3-1: PRACH Mask Index values >