Ok, get a towel and dry your ears
A modulator takes a Video/L&R Audio signal (RCA type) like from your SAT or VCR, DVD, etc...and converts it to a RF (coax) signal, then it modulates it to a specific channel (ie. 90, 92, 94, etc...) when you attach a TV to the coax line you just turn the TV to the modulated channel and..BAM! you are watching your SAT signal. This only works if your home already has cable line run through it....does it?