Hull speed or displacement speed is the speed at which the wavelength of a vessel's bow wave is equal to the waterline length of the vessel.  As boat speed increases from rest, the wavelength of the bow wave increases, and usually its crest-to-trough dimension (height) increases as well.  When hull speed is exceeded, a vessel in displacement mode will appear to be climbing up the back of its bow wave.
From a technical perspective, at hull speed the bow and stern waves interfere constructively, creating relatively large waves, and thus a relatively large value of wave drag. Ship drag for a displacement hull increases smoothly with speed as hull speed is approached and exceeded, often with no noticeable inflection at hull speed.
The concept of hull speed is not used in modern naval architecture, where considerations of speed/length ratio or Froude number are considered more helpful.