A technology for mounting electronical components
onto printed circuit boards
are drilled into the printed circuit boards. Then the electronical components' wires are stuck through these holes and soldered thight on the other side of the board.
A simple and effective way of mounting components if you plan to do it manually, but in contrast to the SMD
technology, it's harder to build machines that can do it, therefore it's not useful for mass production
. Nowadays it's hard to find printed circuit boards with through-hole technology, because SMD has nearly completely replaced it.