The substrate is the 'bottom layer' in your fish tank, which can just be gravel, or can also be a "nutrient substrate" under the gravel for slow-release fertilisation of the plants.
Gravel is an important part of any aquarium aquascape. It covers the base of the tank and is like a 3D blank canvas for you to decorate. It gives a supportive base to hold your feature-pieces of rock or wood in position and should be the right texture to encourage good root growth for your plants.
What goes on top?
The quartz gravel is perfect for growing plants in - the grain size is 1-2mm so water can still move through it for gas and nutrient exchange and roots can grow through it without getting damaged or restricted. Chunkier-grade gravel is fine if you aren't growing plants, but you will find it harder to keep clean as waste gets trapped in the large gaps between the grains.
How about using sand?
We would advise you to avoid fine sand as the particles are too close together so it compacts over time, squeezing all the oxygenated water out. This means any bacteria activity under the gravel has to be without oxygen, so you get colonies of anaerobic bacteria, which means when you disturb the sand you get those swampy bubbles belching up through the water - nice! Sand also collects waste on it, which will drift around like tumbleweed and pile up in dead-spots of current, e.g. at the base of plants or rocks. This looks awful and will need regular vacuuming to keep it looking good (and the sand easily gets sucked up the siphon tube and gravel vacuum so you'll end up with a bucket full of sand and a bare-bottomed tank!)
How much do you need?
To get a good enough coverage for healthy plant growth, aim for about 5cm depth at the front of the tank, banking up to 10cm at the back. Roughly you'll need 5Kg per 30cmx30cm base area. If you really want plants to do well, add the AquaBasis substrate fertiliser under the gravel - now is the time before the water goes in!
How about under-gravel heating?
This isn't essential but you'll get more lush and consistent plant growth. It's a low-wattage heating cable that snakes through the substrate. As it gently warms the water around it, the warmer water rises and carried nutrients up towards the top layer of gravel where fine-rooted plants feed. As this warm air current moves up, cooler water is pulled down to take its place, which in turn gets warm and rises, so you get constant cycles of convection currents for constant exchange of gases. This benefits the bacteria in the gravel (which will improve your water quality) and keeps the plants supplied with what they need.