An example of a flow resource would be either the wind, tide or solar power. Anything that replenishes itself without human intervention is considered to be a flow resource.
Flow resources can only be natural resources because they come from the Earth. Humans have no impact on the process, except for collecting the resource. Unlike renewable and non-renewable resources, which can be replaced or have finite usage, flow resources will be available forever so long as there is no environmental catastrophe that would cause a sudden change in nature. Flow resources can be used without fear of harming the environment or ecosystem.