Net Pots, also known as net cups,, are a traditional hydroponic medium container. The purpose of a net pot is to allow maximum oxygen uptake by the roots. Net cups also allow roots to grow out of the sides and bottom of the container. In order for the roots to grow out of the container they need to be in a dark enclosed growing environment. Examples of enclosed growing environments are Deep Water Culture(DWC), Nutrient Film Technique(NFT), and Aeroponics.
Mesh pots come in various sizes and are perfect for DIY hydroponic systems. They usually come with a small lip that will catch the sides of the container with the hole where the net pot goes. The homemade hydroponic system will hold the net cups in place to prevent them from falling in the reservoir. If you plan to use your net pots as plant containers for consumables, make sure to get food grade plastic.
There are also net pot bucket lids you can get. The lids fit on standard 5 gallon buckets. The bucket lid net pot allows you to turn a bucket into a homemade DWC or ebb and flow system. The mesh pots can be filled with different kinds of medium to fit your needs as a grower. In the picture I have a net cup filled with rockwool and hydroton clay pebbles. Try experimenting with different mediums and mixes to find out what works for you.