To elaborate on Bill's post for non-hams, HF is radio below about 30 MHz (near 10 meters wavelength and longer), usually single sideband or AM these days, but may be other modes (especially CW, or morse code), and tends to be more subject to electrical interference than the much higher freqs and other modulation modes such as FM. And when a ham is listening to HF, many signals are extremely weak, such that it doesn't take a lot of electrical "noise" to hide the desired signals.
Many electrical devices generate electrical noise of one kind or another, especially in these digital, computerized days -- for example I have a food saver (vacuum packing appliance) that generates quite a bit of that "noise" when plugged in, whether actually turned on or not, so I must leave it unplugged when not in use.