What are uniform and non-uniform flows?

In uniform flow, the magnitude and the direction of the flow are constant.  In non uniform flow, the magnitude and the direction can change from time to time.

The flow of liquid through a pipeline of constant cross section can be uniform as the velocity will be constant throughout the line.

On the other hand, if the pipe has different diameter at different sections, the velocity will not be constant and the flow is said to be non-uniform.

A steady flow can be uniform or non-uniform.  Similarly, an unsteady flow can also be uniform or non-uniform.