It denotes the percentage of successful connections. In general, connectivity is calculated by the following formula:
connectivity = 100 - (TCP sessions refused in [%] + TCP sessions timeouted in [%])
These are all sessions which get to the target machine but for some reason are refused and ends with TCP packet with RST (reset) field set. There are couple of reasons why connection is refused, but mostly this is either due to not opened port or blocked port by firewall on the target machine.
Formula: sessions refused [%] = reseted sessions received / new sessions received * 100%
Those are connections without any response from the target machine for given period of time.
Formula: sessions timeouted [%] = timeouted sessions received / new sessions received * 100%