A small crack or ledge which one can stand on while climbing. Not as good as a foothold, but it should be able to support you.
'Toehold' is often used metaphorically to refer to a small, but usually significant, advantage. It is usually a precursor to a more useful advantage or position. A small amount of power, or territory, or influence.
A toehold is also a wrestling hold, but that is sufficiently covered in the toe hold node.
Sometimes shorthand for toehold purchase.