What do you mean by specific heat capacity of water is 4200 J/kg°C?

The specific heat capacity of water is 4200 J/ kg°C means that 4200 J heat energy is required to increase or decrease the temperature of 1 kg by 1°C . 

Specific heat capacity is defined as the amount of heat required to raise the temperature of 1 kilogram of a substance by 1 kelvin (SI unit of specific heat capacity J kg−1 K−1).The specific heat capacity of water is 4.18 J/g/°C. We wish to determine the value of Q - the quantity of heat. To do so, we would use the equation Q = m•C•ΔT. The m and the C are known; the ΔT can be determined from the initial and final temperature.For liquid at room temperature and pressure, the value of specific heat capacity (Cp) is approximately 4.2 J/g°C. This implies that it takes 4.2 joules of energy to raise 1 gram of water by 1 degree Celsius.

International system. The SI unit for heat capacity of an object is joule per kelvin (J/K, or J K⁻¹). Since an increment of temperature of one degree Celsius is the same as an increment of one kelvin, that is the same unit as J/°C.An object's heat capacity (symbol C) is defined as the ratio of the amount of heat energy transferred to an object to the resulting increase in temperature of the object. C=QΔT.Molar heat capacity is a measure of the amount of heat necessary to raise the temperature of one mole of a pure substance by one degree K. Specific heat capacity is a measure of the amount of heat necessary to raise the temperature of one gram of a pure substance by one degree K.

The SI unit of specific heat is joule per kelvin and kilogram, J/(K kg). For example, at a temperature of 25 °C the heat required to raise the temperature of 1 kg of water by 1 K is 4179.6 joules so the specific heat capacity of water is 4179.6 J. kg. K⁻¹.