Question on oscillator

Without knowing the context it is impossible to give a specific answer but in general "weak" means low power. RTCC in particular are usually running continuously with battery backup so keeping current and therefore power consumption as low as possible is required. It implies the output of the oscillator is buffered if it has significant load but the buffer may be powered down if not needed at the time, for example when the RTCC has to run but whatever it drives is disabled or powered down. Typically, that schematic configuration is used where there is an inverter inside the device between X2 and X1 and the crystal with the capacitors provides the necessary frequency and phase shift to meet oscillation conditions. I would guess "Rbias" here is to provide a start-up kick or maybe hold the inverter in a mid logic state so it tries to behave in linear mode but its exact purpose would only be known to the device manufacturer.

Brian.