With your camera and ND filter, you can capture motion and stillness. It gives you a 'fluid - blurry - organic' feel to your shots, which are always static in nature.
This exercise helps you to better understand your camera. We take static shots daily. What if the shutter speed is slowed down intentionally? The picture would be overexposed. What if you compensate for over-exposure and still slow down the shutter? You'd get correct exposure but blurry movement in your photos. How much blurry movement do we want? A lot? A little? That's where we experiment with the shutter open for 1/2 second, 1 second, 2 seconds and see what we discover in our shots.
You might find yourself going as slow as possible (in terms of shutter speed settings), after stopping your lens aperture all the way down and choosing the slowest ISO you have, yet still not getting a slow enough exposure, like maybe if the sun is too bright.
In such cases, you need a Neutral Density Filter (ND Filter). It's like sunglasses for your lens. When your camera detects dark conditions, the shutter speeds slow down.
Google for ND Filters and find out what's ND2, ND4 and what all the numbers mean.
What you need to bring:
ND Filters of various densities (I use a variable ND Filter)