A foothold is a flat or steady area on which you can put your foot when you're climbing. Part of the trick to rock climbing is learning to find good footholds.
A literal foothold is wide enough for you to fit your foot and sturdy enough to hold your weight, and a figurative foothold gives you this same sense of security, usually setting you up to make progress. For example, your first published magazine piece can give you a foothold in the field of freelance writing. This sense of foothold dates from the mid-17th century, meaning "stable ground from which to act."