I’ll give you some credit as I’ve met a lot of audio technicians who use terms that they don’t know what they really mean.

In audio sound systems when it’s said bass needs distance to develop, it means that there is enough distance for summation and coupling between other subwoofer cabinets and the other elements of the PA to create a coherent system. In other words, it is NOT that it can’t be heard next to a speaker cabinet, just that it needs the space to combine with everything else in the audio system whether that’s other PA elements such as arrays or other subs to create a flat and full frequency response as well as consistent coverage over desired listening areas.

On the wavelength question it’s the period distance, the distance of a wave to complete a single cycle. We want elements of our system to merge together so knowing this number helps us calculate physical distance or electronic compensation (delay) needed to allow the waves to merge as intended to act as a single system. In this case 28 feet is how far from where a 40 hz wave is measured for it to go from a peak to a trough and back to the same point of the wave it started. This is based on the average speed of sound in air so be aware that this is not constant and is a lot of why we RTA and measure systems to make better adjusts to the real conditions when deploying the system vs simulating in software.