A tap is a wire coming out of the transformer.
A transformer is two wire coils wrapped around a common core. Usually, the primary side will have only two wires coming out, positive and negative, with maybe a third for ground. That's also called a center tap. The secondary is wired so that you can use the whole coil, which will match a 4 ohm load to the output impedance of the tubes, or half of the secondary coil, for 8 ohms, or a quarter, for 16 ohms. These are called taps, because you "tap into" the secondary at different points.
The different taps were used to match to different speaker cabinets. normally, if the amp came with an 8 ohm cabinet, it would be wired 8/4, so you could run one cab from the main output, or get a second cabinet and run two, one from each output. Likewise, if it came with a 16 ohm cabinet, it would be wired 16/8. I can't say how consistent they were about that, though.