'Upres' increases the resolution of the simulation without a large increase in simulation time. Compare these two simulation, using the default settings but with 'Upres' disabled in the first and enabled in the second:
Note the considerable increase in detail in the second video.
The tab interface looks like this:
Check this switch to enable the upres feature.
The voxel size used for the upres simulation.
Vorticity, Turbulence Strength, Turbulence Radius
These parameters are the same for the upres part of the simulation as found in the Simulation tab.