I've heard that to receive benefits you have to being actively treated. What does this mean?

Most disability insurance policies do require that you be under the current care of a physician.  The term "actively treated" of course has different meanings for different illnesses.  At some point the insurance company may require you to be seen by an independent medical doctor to see whether the treatment you are receiving from your doctor is appropriate.  With some diseases/illnesses, periodic checkups at six month intervals is appropriate.  Each case and each situation is different.  If you run into problems with your disability insurance company you should seek the advice of an experienced disability insurance attorney.