Naive Question: What Does it Mean to Say a Sub is "Tuned" to, for example 20 hz?  

Hi - I was reading a post about "sonotubes" and saw reference to a person's sonotube which was "tuned" to 18.5 hz. What does this mean? That it will play only 18.5 hz? That it will play cleanly / level down to 18.5 hz? Something else?

Thanks