What is a 2 ohm receiver?

A receiver is a device in car and home audio that contains a pre-amplifier, amplifier, (usually) a tuner, and (sometimes) equalisation settings.

A 2-ohm receiver is one that can comfortably supply power to a 2-ohm load (i.e. the speaker). A 2-ohm speaker is relatively non-resistant load, meaning that it will take a lot of CURRENT necessary to FLY through it to operate it satisfactorily.

If you tried to power a 2-ohm load with an 8-ohm receiver, for instance, the 8-ohm receiver would be pushing tons of current through the speaker, but the speaker would not physically be moving very much (perceived as low volume), because not enough work is being done in the voice coil. If you attempted to raise the volume up to listening level, you run the risk of overtaxing the power supply, and thus destroying it.

In your situation, however, only seldom are there speaker products rated lower than 2-ohms, so you probably shouldn't have to worry about that. I've seen large subwoofers rated at 1/2 ohm, or 1 ohm, but these are a speciality item, and you'd probably know if you had one.