the colon after the long is mean what?

This is a bit field (You can use Google for more information and samples).
 
Essentially, it means that 2 bits are used for a1 and 30 bits for a2. 64 bits are used for a. This give a total of 96 usable bits. Actual size might be bigger for alignment purpose.
 
Fot the bit fields themselves, they will generally be combined up to the size of an int and if more bits are required, the will start on next boundary. Also, the maximum number of will usually be 32 or 64 depending on the platform.
 
Thus a bit field can essentially be used to split a type into multiple bit fields. Thus is can be used to reduce memory usage in some case or also simply to define a structure that matches somne hardware registers (more common in Embedded software).