How infrared radiation are helpful in keeping our earth warmth and in physical therapy?

Dear student, 

Infrared rays are heat rays. They are responsible for transferring heat. The heat reaching us from the sun are the infrared rays of the sun. As such, they are responsible for keeping the earth warm.
Many physical therapy are based of heat application. In such cases infrared rays are involed.

Hope this clears your doubts.
Regards