I'm no technical guy, but Onemug is right. A tube amp, other than an OTL (output transformerless) design, uses an output transformer to match its output with the impedence load your speaker presents. A solid state design generally does not use an output transformer and thus does not have different taps. The taps represent different points in the transformer windings (a transformer is wire wound around a core)that are optimal for the load in question. Outside of that I can't tell you too much, I'll leave it to a more technically inclined poster to correct me/elaborate for you.
By the way, I noted another post where you were asking about speakers to go with your new amp--don't be too anxious to change the Shahinians quite yet until you listen to them a while with the Mac, you might be able to resist the upgrade bug. The Shahinians are similar to my speaker system in terms of being closer to an omnidirectional speaker than the other, forward-firing designs you mentioned in your other post, and as such they present music in a significantly different manner than those other speakers. If you like and are accustomed to the Shahinian "sound", you might have trouble listening to more conventional designs. The other designs may do some things far better, such as pinpoint imaging and presentation of low level detail, but the overall listening experience might not be as pleasurable.