A streamline is a field line in the velocity field of a fluid at some instant, ie. a curve which on all points along it is in the same direction as the velocity of the fluid at that point and time.
In a steady flow, ie where the velocity field does not change with time, the streamlines show the paths of the fluid elements (it is worth noting the difference that a streamline is a curve in space while a particle moves in space-time, and that its path is merely a projection onto space). In an unsteady flow this is not the case, and the streamlines will also change with time. In any case streamlines give a good view of the instantaneous properties of the fluid motion, in the same way that magnetic field lines help visualise magnetic fields etc.