A semiconductor is a material which only conducts electricity under certain conditions. They are a number of these materials in common usage, the most ubiquitous being silicon and germanium.
The term is usually used to describe a device made using this technology.
It is by taking these semiconductors and “Doping” them (introducing an impurity) that you can create “p” and “n” type semiconductor, one being populated with “holes”, and another with free electrons. If you place a slice of P type, beside a piece of N type, the current will only flow in one direction across the transition area, and only when there is enough current to reach the threshold for a flow.
By placing three slices - PNP or NPN you get a transistor, and by putting the correct current at the inner slice, you make the charge between the two outside types “jump” across and allow current to flow.