A solution that contains all the solute that it can hold is a what solution?
This is called a saturated (or eventually a 'super-saturated') solution
Under some circumstances it is possible to dissolve more of a solute into a solution than the nominal solubility of that solute would allow. A saturated solution is one that contains all the solute that will normally dissolve, and a supersaturated solution contains even more of that solute. If the solution is disturbed in some way (the appearance of a nucleating particle, stirring, etc.) then the excess solute will precipitate from the solution, or in…
A saturated solution contains all the dissolved solute that it can have in a stable state; a supersaturated solution has more dissolved solute than the amount that constitutes a saturated solution, and as a result it is not stable. If the solution is perturbed in some way, there will be a precipitation of the solute.
A solution is a solute dissolved in a solvent. A concentrated solution is all the solute that be dissolved in a solvent at normal temperature. A super-concentrated solution is all the solute that can be dissolved in a solution after mixing in the solute during high temperature / pressure. The concentration after cooling to normal temperature / pressure is greater than a regular concentrated solution.
It is a supersaturated solution. These solutions are formed when a saturated solution with excess solute in it (like saltwater with extra salt on the bottom) is heated until all the solute dissolves. When the solution is cooled, the extra solute remains in solution--thus supersaturated because more solute is dissolved than should be at that temperature. The seed crystal provides an opportunity for the extra solute to come out of solution.