Pure silicon is an insulator, but can be made to conduct if certain other atoms are diffused in to it.
One side of a semiconductor junction has atoms with more electrons that silicon (N type), and the other side has atoms with less electrons (P type) with leave "holes" in the crystal matrix. That's a P-N junction. It forms a "depletion layer" in the centre that does not have any charge carriers.
With positive and negative the right way around, electrons and holes are pushed in to the otherwise insulating depletion layer and the junction conducts.
The other way around the electron are pulled away from the pure layet and there are no charge carriers in it, so it does not conduct.
A transistor has two P-N junctions back-to-back.
When the base-emitter juction has current passed through it, the charge carriers that junction injects to the silicon allow the overall device to conduct.