TV-What is a RF emi cable & what does it look like?

I have an Insignia 32 inch advanced serious(NS-L32X-10A) and the user's guide book says use a rf emi cable(coaxial) but there is no cable connecting to the TV,I do have a Com cast basic HD box(no ons on it and you have to use your remote to turn it on and off) and there is a coaxial cable going from the wall to the back of the HD box(the coaxial cable says Gilbert on both ends) and there is an HDMI cable going from HDMI 3 on my TV and the other end is connected to the HD box