What is the use of the RAT on the aircraft and is it limited to specific speed?

Ram Air Turbine (RAT) is a small turbine being used for emergency purposes. Generated power from the air stream by ram pressure due to speed of aircraft. RAT is not limited to specific speeds, If the speed of the aircraft is low then the power will be low too and vice versa.

The RAT (Ram Air Turbine) in an aircraft is used in Emergency purposes. Its used in case of system failures.It has two blades comes down of the belly of the fuselage to generate minimum hydraulic and electrical power to keep aircraft lands safely.And it has no limitations it depends on air stream velocity.

The RAT(Ram Air Turbine) in an aircraft is used in Emergency purposes. Its used in case of system failures such as hydraulic or electrical system fail.The power it generates solely depends on Ram Airspeed, Hence it is not subject to any specific speed limit.

The Ram Air Turbine (RAT) is at the heart of an aircraft’s emergency power system. In extremely rare instances when airplanes lose power, the ram air turbine deploys from the airplane's wing or fuselage and rotates to extract sufficient power from the airstream to control and land the aircraft. The business designs and manufactures hydraulic, electric and hybrid RATs.

Rat is used in case of emergency after IDG & APU electrical generation failure and when the speed is above 100 knots the RAT is deployed automatically in case of failure it can be deployed manually its purpose is to used the wind energy to generate power to supply basic aircraft systems 

I'm not an engineer but I have taken a subject called Aircraft Design and Performance during university and RAT was my research topic for the course!  hope this would be simple yet helpful:

a Ram Air Turbine is a small airflow driven engine that supplies power to the electrical or hydraulic system of an aircraft during an emergency by running a propeller and generating power from the free airstream. 

by emergency I mean total and complete power loss due to multiple engine failure.

it is found in almost all commercial aircraft and can be automatically or manually deployed only after a complete power loss. 

RAT power output reduces as the aircraft speed drops and vise versa. 

The RAT generates power from the airstream by ram pressure due to the speed of the aircraft.

The power generated depends on the speed of the aircraft.