Short-tail keywords are broad, one- or two-word terms, "shoes," "SEO," "insurance", that carry the highest search volumes of any keyword category precisely because they're so general. That breadth is also the problem: a term this open-ended can mean a dozen different things depending on who's typing it, which makes user intent genuinely hard to read from the query alone.
That ambiguity, combined with the sheer volume at stake, is exactly why short-tail terms attract the fiercest competition in SEO: every site with any interest in the category is fighting for the same handful of top positions. Ranking well for one can still bring in enormous traffic, which is why they remain worth pursuing for established, high-authority sites, but for a newer or smaller site, that competition usually makes short-tail terms a poor place to start. The more realistic path is building authority and traffic through the long-tail variations underneath a short-tail term first, then working toward the harder, broader term once a site has earned enough trust and links to genuinely compete for it.
