A toehold is a sturdy spot where you can put your foot, especially when you're climbing. If you don't find a safe toehold, you'll never manage to get over that fence to get your baseball back.
A literal toehold is an area where at least part of your foot can fit and that's strong enough to hold your weight. A figurative toehold is a tiny advance that gives you a small start toward progress. Getting an internship at a publishing company will give you a toehold in the industry. The literal definition, "support for the toe of a boot in climbing," dates from the late nineteenth century.