I would suspect that instead of word frequency, they likely use TF-IDF (or even better, BM25[1]) or LDA [2] to identify relevant keywords to highlight.<p>1. <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Okapi_BM25" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Okapi_BM25</a><p>2. <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Latent_Dirichlet_allocation" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Latent_Dirichlet_allocation</a>