– The Lattice Boltzmann Method (LBM) used by Blender to simulate the evolution of a fluid stores average values over each cell (Voxel).
– Tracer particles are to track the evolution of the fluid using discrete particles in well-defined positions and independent of the dimension of the Voxels.
– Unfortunately at this moment tracer particles are not always behaving nice, some stick to the walls of the domain and seem to be dead. I hope blender developers fix this soon!
– To create tracer particle add another object with the Particle Fluid type, that uses the same bake directory as the Domain.

– In this example you can see how tracer particles follow the motion of the fluid. You can also see lot of particles sticked to the left side of the domain (Domain Boundary Type was set as Free-Slip so no particle should stick to it!).

Blender Fluid Tracer