What does it mean by Altimeter is calibrated to ISA conditions?
Does it mean at a sea level with 15C deg and pressure of 1013 hPa the altimeter will read zero?
Vitally, it means more than just that. As speedrestriction correctly stated it means that the altimeter will read correctly throughout an ISA. Quite possible to have an altimeter which could still read zero with 1013 set, but if not calibrated to ISA it would then under/over-read as it was moved away from sea level. Ie, when indicating FL400 it could well have significant errors despite correctly reading zero at sea level.
Aviation altimetry relies on accurate calibration throughout an altimeter's operating range and not just at one point.