Smith machine

In plain words

Hooks on the bar catch on the frame when you twist it, so you can park the bar at any height. Wikipedia describes it as a barbell fixed within steel rails allowing only strict vertical movement; some machines also counterbalance the bar.

Two things catch beginners. The bar is not 20 kg, and how much it weighs differs by machine, because counterweights and the carriage change it. The same page notes that makers do not usually mark the bar weight on the machine. And because the path is fixed, a Smith machine squat is a different exercise from a free barbell squat: the numbers do not carry across.

An example

Say your gym's machine has a starting resistance of 13.4 kg, which is 30 lb, the figure Life Fitness lists for one of its Smith machines. You load a 20 kg plate on each side. The lift is 53.4 kg, not the 60 kg the same plates would make on a free 20 kg bar. Log it as a Smith machine press, not a bench press.