Walking on the Slack Rope is a gymnastic skill, like Tight Rope walking, and is often used by acrobats, clowns and comedians as a circus or vaudeville act. According to Bosman, 1928: p.227), it was particularly popular in the mid-1800s in South Africa. See for example Monsieur Dupree (1833).
A modern form of the slack rope walking is an "extreme sport" version, which arose in the late 1970s, and the activity is today called "Slacklining".
See also: Tight Rope walking (or funambulism).