Winterization is a process of purification by fractional crystallization.
Winterization is the process of "freezing" a solution of hash oil in ethanol.
Hash oil contains THC plus waxes, chlorophyll, and plant material.
A solution of hash oil in absolute (95 %) ethanol is placed in a freezer for one to two days.
The less polar waxes and plant materials precipitate from the polar ethanol, and the more polar THC stays in solution.
The solids are removed by filtration, and the ethanol is removed by natural or vacuum evaporation.
You can find fairly detailed procedures by entering "How to winterize BHO" into your favourite search engine.