What does this equation mean?

The Text REtrieval Conference, run annually by the U.S. National Institute of Standards and Technology from 1992 onward, gave information retrieval something the field had mostly lacked: a shared, blind evaluation on a common document set, repeated every year with published results. Robertson, Walker, and colleagues at City University London entered TREC-3 in 1994 with the Okapi system, reporting a term-weighting approach within a probabilistic relevance framework and applying it, among other extensions, to phrase weighting and to query expansion using terms drawn from an initial pilot search [ 2 ] . Over that and the following TREC rounds, the Okapi team’s tuning of term-frequency saturation and document-length normalization converged on the specific scoring function that the field came to call BM25 — “Best Match 25,” after its position in a numbered sequence of variants the group had tried. Written in its now-standard form, a document D scores against a query Q as $\mathrm{score}(D, Q) = \sum_{q_i \in Q} \mathrm{IDF}(q_i) \cdot \frac{f(q_i, D) \cdot (k_1 + 1)}{f(q_i, D) + k_1 \cdot \left(1 - b + b \cdot \dfrac{|D|}{\mathrm{avgdl}}\right)}$. where f($q_i$, D) is the frequency of query term $q_i$ in D , |D| is the document’s length, $\mathrm{avgdl}$ is the average document length in the collection, and $k_1$ and b are tuned constants controlling term-frequency saturation and length normalization respectively. The saturation term is the substantive advance over a raw term-frequency-times-IDF score: a term’s contribution grows quickly at first and then flattens, so a document that happens to repeat a query word fifty times does not dominate one that uses it three times in the right place. Decades later, this same function remains the default first-stage ranking method built into widely used open-source search engines, which is a strong claim to make about any piece of 1990s software and is offered here as an observation about longevity rather than as evidence that nothing since has improved on it.