I am new to gardening in a greenhouse and I would like to start lettuce and tomatoes in a soilless medium but I really don't understand what a soilless medium is or how to go about making the soilless medium.
Certified GKH Gardening Expert
This article should also help: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soilless-growing-mediums.htm
Soiless medium is just that, there is no regular soil used in the mix. There are advantages to using this mix when you are starting seed, you have better control of what's in the mix, with less chance of contaminates or diseases and fungi that could be present in regular garden soil.
You can either buy the mix or make your own. The following site has some excellent suggestions for recipes for making your own. I have used several of them with great success.