A Smith Machine is a piece of gym equipment that consists of a barbell fixed within steel rails or guides, allowing for vertical movement. It's designed to assist with weightlifting exercises by providing stability and controlled motion. 

It's important to note that while the Smith Machine offers certain advantages, it also has limitations. Because the barbell is fixed to the rails, it may not fully mimic the natural movement patterns required for certain free weight exercises. As a result, some advanced lifters prefer free weights for compound movements.