Question

what is meant by vegetative propagation by stem?

Answer 1

vegetative propagation is the propagation of a species by making simple copies of itself, sometimes through a stem, sometimes through the roots, or leaves, or bulbs.
Advantage is that the plant does not need another plant nearby to reproduce (like in pears). However, since both the original plant and the new plants have the same genetic components, there will be no adaptive evolution advantage except for random mutations.

See http://leavingbio.net/vegetativepropagation.htm
for examples of different kinds of vegetative propagation.

Answer 2

thank you so much for your help MATHMATE