This means that most real systems are not $100\%$ efficient. The efficiency of a system is the percentage of input energy/work (or power) that gets transformed into useful output energy/work (or power). This is given by: \begin{align*} \text{Efficiency}&=\frac{E_{\text{out}}}{E_{\text{in}}}\times 100\%\\ &=\frac{W_{\text{out}}}{W_{\text{in}}}\times 100\%\\ &=\frac{P_{\text{out}}}{P_{\text{in}}}\times 100\% \end{align*} $W_{\text{in}}$, $E_{\text{in}}$, $P_{\text{in}}$ and $W_{\text{out}}$, $E_{\text{out}}$, $P_{\text{out}}$ are the input and output work, energy and power respectively.