The R in a circle symbol next to an airport on Sectionals nindicates
that this airport has Airport Surveillance Radar. But what does this
really indicate to the VFR pilot?

At Reading, Pennsylvania (KRDG) there is such a symbol. This (Class D
Airspace) airport has its own Approach control (shared position in the
tower, actually). Unfortunately at no place on the Sectional is a
frequency for approach given, even on the tables next to the chart.

At Nantucket, Mass. (KACK) there is such a symbol. This (Class D
Airspace) airport does not have approach control. Radar services are
provided by Cape Approach during the day and Boston Center during the
night. (Active times are not given, but there are boxes on the map
itself that indicate what frequency to use for Cape Approach).

Stewart Intl, NY (KSWF) also has such a symbol. Radar services are
provided by New York TRACON.
(all of these airports are on the NY Sectional).

So what does this symbol really mean? At one airport where it is used,
the airport has a hidden-from-the-chart approach frequency. (The
frequency is of course available in the Airport/Facility Directory and
IIRC via ATIS among other places). At other airports, radar services
are provided by facilities that also provide service to many other
airports (without the R symbol.)

The only thing that I can think of is that it indicates there is a radar
(ASR) antenna on the field for a class D facility. Then again, Class D
airspace airport KMDT (Harrisburg Int'l) has such an antenna (atop the
hill nearby), but no such R in a circle icon on the sectional. It does
however had a surrounding TRSA with a TRACON, so perhaps that is why no
R in a circle is necessary.