In BiCMOS technology, bipolar transistors are integrated with CMOS transistors within a single integrated circuit (IC) device.  In integrating these two different technologies, it is generally desirable to build a bipolar device that performs at fast speed.  An important figure of merit to determine whether the device performs fast enough is the maximum frequency. 
The maximum frequency of oscillation (fmax) is the maximum frequency for the transistor where the power gain is equal to one.  A faster transistor will have a high fmax.  In order to produce a faster transistor, the base resistance must also be lower.