Crystallization is a method to obtain pure solid from impure solid. An impure solid dissolves in a liquid(solvent) to produce a solution. The solution is then heated up.
When most of the solvent is evaporated off, the hot solution is left to cool down. The dissolved solid will appear as pure crystals.
The impurities remain in the solvent. Then, the cold solvent is poured off to obtain the crystals.
Filtration followed by crystallization is often used to obtain pure salts such as copper sulfate. Sugar is obtained by this way too.