On the Solver tab different solvers for velocity and pressure correction equation can be selected.
Usually the defaults should not be changed.
Item  Description 

Velocity solver and Preconditioner  Determines which solver will be used, see Linear system equations solvers , default is STABBiCG. The default preconditioner is ILU. 
Pressure solver and Preconditioner  Determines which solver will be used, see Linear system equations solvers , default is GSSV. The default preconditioner is ILU. 
parameters  Specifies the parameters for the selected solver, left hand side for the velocity solver, right hand side for the pressure solver:
