Micropropagation is the growing of small plants from pieces of tissue using sterile laboratory techniques. This produces clones.
A plant with desired characteristics is chosen, and a small piece called an explant is taken.
The explant is sterilised and then put into a growth medium containing everything the plant needs to grow (sucrose, amino acids, minerals) and plant growth hormones.
The roots will appear first, and then the shoots as the small plant (called a plantlet) develops.
The plantlet is said to be grown in vitro which is latin for in glass. So anything grown in a laboratory is called in vitro.