What does it mean to calculate hardness in terms of ppm of calcium carbonate? PPM means per part million or milligram per litre of solution.Calculation of ppm for a given compound say $x \pu{gms}$ of $\ce{Ca(HCO_3)}$ in 1 liter of water would be done like   

$$ppm= \frac{1000\cdot x}{1\pu{1L}}$$  

But I do not understand by what's meant by in terms of $\ce{CaCO3}$ is it equivalents or weight? This was asked in an exam and the formula is not covered in my textbook.
It is meant as molar equivalent. $\ce{CaCO3}$ has the molar mass about  $\pu{100 g / mol}$, $\ce{Ca(HCO3)2}$ has the molar mass cca $\pu{ 162 g / mol}$. Therefore $\pu{100 g}$ of the former is equivalent to $\pu{ 162 g }$ of the latter. The similar for the concentration expressed in ppm. E.g. if water contains $\pu{1.62 ppm}$ of $\ce{Ca(HCO3)2}$, it  is equivalent to $\pu{1 ppm}$ of $\ce{CaCO3}$