A ‘slug’ is a short label for something, containing only letters, numbers, underscores or hyphens. In online surveys, the term ‘slug’ is used to refer to the survey short name, which is the final, customisable part of a survey’s public URL.
The survey short name can be edited in the survey’s 
Distribution settings (on the Distribute page).
If you attempt to change your survey short name to something that contains spaces, special characters, or other characters that are not able to be used in a URL, online surveys will prompt you to enter a valid ‘slug’ consisting of letters, numbers, underscores or hyphens.
NB: The use of the term ‘slug’ in this context is likely to refer to prior use of the term in the printing industry.