Hemp seeds are a secondary product when collecting hemp plants. These seeds can be planted in the ground and grown to collect additional cloth. A common practice is to plant them immediately after harvesting out in the world so that other new players can get cloth from it later on.
If they are planted in the ground, watering them has no effect, and you are not able to use fertilizer on them. This means that they are lower maintenance, but you have no control over the quality of the soil, so they will typically grow more slowly.
However, if you plant them in a planter box (small or large), they will require you to regularly water them and keep them stocked with fertilizer for maximum growth efficiency. Be careful not to over-water them, or efficiency may go back down a bit. Generally a good amount of water to have is somewhere between 50% and 70% capacity.
The plant needs light to grow, whether from a lamp or the sun. As of May 2020, floor grates do not let light through to the plant, despite looking like they do. This is likely a bug and may be fixed in the future. With the new electric water pump and electric water purifier, it is a good idea to build either on the beach or on a river (with one lower or missing foundation for safe access to water), as you can set up automatic plant watering fairly easily.