An antenna's radiation efficiency is pretty simple to understand. It refers to the difference in the total power inputted into the antenna and the total output power radiated. Any losses in between these steps are due to electronic losses. More formally, if we consider an antenna in transmitting mode, the radiation efficiency is given by $\epsilon_R=\frac{P_{radiated}}{P_{input}}$.
A related term, the "total efficiency" of an antenna is given by the formula $\epsilon_T=\epsilon_RM_L$, where $M_L$is the impedance mismatch loss of the antenna (see section in Antenna Theory Hub).
Typical efficiencies for antennas can range from 20%-70% for everyday cell phone antennas, to very close to 100% for dish antennas.