A classic workhorse typeface, a workhorse typeface or just a workhorse are descriptions given to a font tool that performs dependably (i.e., trustworthy and reliable) under heavy or prolonged use.
In a nutshell, a font is a workhorse if it performs well whether it is used in a few paragraphs or in a 600-pages novel.
A workhorse- or “easy-to-use”- type means that no matter what size, leading, letter-spacing, etc., the setting just “feels” right. It looks professional, it looks appropriate.
To mark a typeface design as workhorse, one should also know what the designer intended, know if the target audience is (over)used to the font, what attitude it conveys subliminally; how dark it makes the page, whether it suits fine at looser or tighter leading and what point size is most suitable for different lengths of copy and for typographic hierarchies (e.g., headings). Also know how the family fits together; how much emphasis does a bold give to the roman, and if the italic is different enough for emphasis.
A workhorse font must has features such as small caps, true italics, extended language support and multiple numeral styles; eventually including technological solutions for setting maths. Monospaced fonts may fit in due to their special metric and screen properties.
Workhorse text faces are often main foundries bestsellers rather than scripts and display.