Hi,
_ReadAttrCB is a callback. You give its pointer to some routine that initializes profile - job's done :)
The OSAL (which is essentially a neat multi-layered while(1) loop that allows RF core to have more CPU time while less timing-sensitive tasks step aside) will then call _ReadAttrCB every time when some device connects to your device and reads an attribute. Of course it will also call WriteAttrCB callback once someone tries to write to your attributes.
Generally, you should not worry about how handle, offset, len and value got there. Treat them as an R/W operation arguments passed along with radio read/write command.
BR,
Oleg