COP is Coefficient Of Performance, basically the heat they produce divided by the electricity input, so a COP of 4.5 means that 1kW of electricity produces 4.5 kW of heat (it's taking heat from the environment so you can say a COP of 4.5 means it's running at 450% efficiency, but that's only in terms of electricity use, not actual overall efficiency - but electricity use is what we care about).

COP is only an instantaneous measurement though, and changes depending on the outside temperature. So if you need heating for five months a year, and it's usually exceeding COP of 5 for 80% of that time but dips down to a COP of 3.0 on the three or four coldest days of the year, it's not really correct to say it's either >5 or that it's 3.0 - so SCOP is used as a 'seasonal' COP that is averaged over a longer time period, so you can compare different systems over the longer term.