Similar to the GET function in HTTP, subscribe is the method to obtain values but with a huge difference: you do not have to be continuously asking the server for each value in your custom script.
For example, if a variable's value changes, Ubidots will automatically update you of any changes. Thus you will save data requests, processing time for your device, and overall project functionality as well as expenses. That's why MQTT is a communication protocol advised to implement firmware routines to control actuators.
The Subscribe Topic structure is as follows:
/v1.6/devices/{DEVICE_LABEL}/{VARIABLE_LABEL}
/v2.0/devices/{DEVICE_LABEL}/{VARIABLE_LABEL}