Lock step refers to any of several dance steps that involves the "locking" of the moving foot: the moving foot approaches to the standing foot, crosses in front of or behind it in the direction of the approach, stops close to the standing foot, and the weight is fully transferred to the (previously) moving foot.
The closeness of the feet so that the lower legs of the dancer are "locked" together crossed is the main difference from other cross-steps used, e.g., in the grapevine, where the moving (crossing) foot travels away from the standing foot and the lower legs are separated.
The footwork varies depending on the actual dance figure.
Lock steps are common in the quickstep.
Among the basic figures of Argentine tango there is a similar "la cruzada" ("crossing) position of the legs.