The % impedance is formally referred to as impedance voltage. It is the supply voltage, expressed as a % of rated voltage, that is required to circulate rated current through the transformer.
It is measured in the factory by a short circuit test at rated frequency. With the low-voltage winding shorted, the supply voltage to the high-voltage winding is increased until rated current flows in the transformer. The supply voltage magnitude that results in rated current is then referred to as the impedance voltage. When it is divided by the rated voltage of the transformer, it becomes the %- impedance voltage, or more commonly, the %-impedance.
For example, if you have a transformer with a positive sequence impedance of 8% stamped on the nameplate, this means that 8% of the rated winding voltage is required to produce rated transformer current. If the high-voltage winding is rated at 220 kV, then 0.08*220 kV = 17.6 kV is required to produce rated full-load current when the low-voltage winding is shorted. This test is typically conducted under normal cooling conditions (i.e., natural air and oil; no forced air fans or forced oil pump cooling).
The positive sequence impedance is determined by a three-phase short circuit test; and the zero sequence impedance is determined by a single-phase short circuit test.
It is also a good indicator of how the transformer will fit into a soft or stiff system, in terms of its fault level and voltage regulation aspects. A system that is weak (high impedance) will be vulnerable to voltage regulation problems and installing a high %Z transformer will make that situation worse. We used to stipulate low %Z transformers (2-3%) at the remote ends of long distribution systems, otherwise starting large motors was problematic (volt drop). High %Z transformers are good to reduce fault levels and associated PSCC, to within the design ratings of the system switchgear etc, on very stiff systems with low impedance and high PSCC levels.