Growing Media is the materials that your plants roots will grow and hold their structure in.
Remember growing cress on those cotton wool pads at school? This is similar.
Growing Media isn’t used much in systems such as Aeroponics, Deep Water Culture or NFT, rather Growing Media is used more in systems like Flood and Drain. The reason it isn’t used in the aforementioned systems is because the roots grow within the growing system and nutrient solution. In a Flood and Drain system, pots are used so the growing media is a more substantial option.
Within these types of systems, growing media is used throughout the plants lifecycle. From seedlings to cuttings, right through adulthood. Plants require this growing media in order to establish their root structure.