A tissue culture is a culture based on a piece of tissue from (the inside of) a mushroom. With a tissue culture you can clone a mushroom.
If you grow mycelium with the help of a mushroom tissue, you can be sure you will get flushes with the same genetic characteristics of the mushroom from which you extracted the tissue. This process is best carried out by using agar. Make sure you extract tissue from the strongest and healthiest mushroom.