There is 6 bits long field in IP header. It is used to make difference between IP packets so router can treat each packet differently. Router interprets DSCP and uses in its routing/scheduling algorithm. Since a number of application data is being carried out by IP layer. Every application want different type treatment (like delay sensitive) . To met such requirement DSCP is introduced.
IP world has done its job for their network but when data comes to LTE network at PGW. PGW has to take care so that IP packet meet such delay budget. That's why for downlink packets PGW check the DSCP of IP packet and send the packets to appropriate bearer. For that purpose LTE has defined QCI values.