One sightline
Follow one row across the field and it becomes a spectrum. Most of the absorption comes from the diffuse hydrogen that makes up the forest. Sometimes the sightline crosses much denser neutral hydrogen in and around galaxies, producing broad damping wings that can cover a large part of the spectrum.
These dense absorbers are often difficult to see directly in emission. Against a bright quasar, however, their absorption lets us probe neutral gas in and around galaxies that would otherwise be hard to see. Measuring them means inferring the quasar light hidden beneath the damping wings, together with the absorber's redshift and column density. Across many quasars, those inferences constrain the absorber population.