The Ohms rating, or "impedance" of a speaker is the electrical load that the speaker presents to an amplifier. A higher number means the speaker has more "resistance" and will draw less power from the amplifier. A lower number will draw more power from the amplifier and make the amplifier work harder.
Most commercially available speakers are rated at an average of 8Ω, 6Ω, or 4Ω. Some amplifiers will only work with high impedance speakers.
Rotel amplifiers will work comfortably with all these specifications. It is recommended to use speakers with a load of 4Ω or more.