Explain what 'selecivity' is in the context of a radio receiver, and describe how a superhet receiver is more selective than a standard radio receiver

Selectivity is the ability for a system to isolate the signal coming from one channel, while filtering out signals from adjacent channels. A selective filter ought to have steep sidebands on its gain-frequency graph, and a flat top.

The superhet receiver is more effective than a standard radio receiver with this due to one of its subsystems, the i.f. (intermediate frequency) filter, consisting of multiple LC filters (also called tuned circuits) tuned at slightly different frequencies, connected in series with buffers at each of their outputs to provide a high output load. Signals within certain ranges, given by the resonant frequencies each tuned circuit in the stacked filter, are then passed through. Using a stacked filter dramatically reduces the impact of signals from adjacent channels (in essence blocks them out) and also gives the gain-frequency curve a flat top, so that any signal within the acceptable range does not vary in amplitude too much.