What does this equation mean?

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.