U-APSD stands fo Unscheduled Automatic Power Save Delivery
It is part of the 802.11e Standard and is related to power saving.
To make it simple, if a network card doesn't have to transmit anything, it will go in standbye mode.
As soon as it has anything to transmit to the AP, the network card will wake up and will transmit the packets.
At the same time, the network card will check if the AP has anything to transmit back.
In case of no traffic, the network card will wake up randomly avery 100 to 200ms to check if there is anything that might come from the APs.
This process is fine for instant messaging, web browsing or mails.
When you need a regular transfert signal, then U-APSD should be disabled.