What a Smith machine is
A barbell on a guided track
A Smith machine fixes a barbell to two vertical rails so it can only travel up and down. Rotate the bar a quarter turn and hooks on the bar catch any of the rungs along the frame, so you can rack the weight at any point in the rep instead of only at the top. Adjustable safety catches set a floor below which the bar cannot travel. Squats, presses, lunges, split squats, rows and hip thrusts all run on the same guided path.
That guided path is the whole trade. You do not have to balance the bar side to side or front to back, which makes the machine easy to learn and easy to train alone near the end of a set. In return, the bar follows the rail rather than the path you would choose with a free bar, so foot position does the adjusting: step the feet forward or back until the movement feels right before you load it.