Answer:
When you see the phrase "in vitro" it means 'in glass'. This is in contrast to "in vivo" which means 'in the living system'.
Thus organic chemistry began when a German chemist made urea 'in vitro' - in the laboratory. Prior to this discovery, it was thought that chemicals/compounds found in living systems, like urea, could ONLY be made by living systems.
In other usage, you may have seen "in vitro fertilization", which means that an egg was fertilized with sperm in a laboratory, rather than in the bodies of living organisms. An important example is the Panda. Apparently these bears have trouble getting pregnant and so many recent baby pandas are the result of "in vitro fertilization", and then implantation of the fertilized egg into the mother panda.
P.S. Laboratory experiments do not always involve the use of glass now, so technically
some "in vitro" work is really "in plastic".