DRX refers to "Discontinuous Reception". It is a technique used in mobile networks that aims to reduce power consumption of the UE.

DRX is used while the UE is in idle state.

In this technique, the UE and the network agree on specific periodic timer (DRX on-duration timer).

When the UE is in on-duration period, it actively listens to paging messages sent from the network, so that it can respond to paging messages sent to it form the network.

While when the UE is in off-duration period, it puts it self in sleep mode in order to save its battery, while in this mode, the UE can't listen to the paging messages sent from the network.

So DRX technique saves the power consumption of the UE, however it may make some delay in responding to paging messages sent to it from the network.