Valet Mode appears to be a method of restricting some of the car’s performance and preventing the valet personnel from using the infotainment center features while they are parking your car. "Steering Wheel Controls" refers to infotainment feature controls that are available on the steering wheel.
As found in the 2017 Volt Chevrolet MyLink Infotainment System manual [Note that the manual covers this feature for many Chevrolet car models, not just the Volt]:
If equipped, this will lock the infotainment system and steering wheel controls. It may also limit top speed, power, and access to vehicle storage locations (if equipped).
Steering Wheel Controls (Volt)
If equipped, some audio controls can be adjusted at the steering wheel.
(Person speaking symbol): Press to answer an incoming call or start voice recognition.
(End call symbol): Press to decline an incoming call or end a current call. Press to mute or unmute the infotainment system when not on a call.
(Left or right arrow): Press the five-way control to go to the previous or next area of a display on the instrument cluster.
(Up or down arrow): Press the five-way control to go up or down in a list on the instrument cluster.
(Check mark): Press to select a highlighted menu option.
(Up FAV or down FAV): Press to display a list of favorites. Press again to select the next or previous favorite when listening to the radio.
The volume switches are on the back of the steering wheel. Press to increase or decrease the volume.