The COP is the Coefficient of Performance. It is the ratio of energy input to output. Eg a unit with a COP of 5 would provide 5 units of heat for every one consumed.
For example a heat pump with a COP of 5 would use about 4kw of electricity and output 20kw of heat (ie 5 x 4kw =20kw).
Bear in mind though that there is no international standard for stating COPs and therefore each manufacturer will state the maximum COP obtainable for their units under optimum temperature and humidity conditions. In reality you may not therefore always acheive the COP rating stated.
This is another reason why you should always oversize your heat pump