Filtration works because the filter paper has tiny holes or pores in it. These are large enough to let small molecules and dissolved ions through, but not the much larger particles of undissolved solid.
Separating insoluble solids
To obtain large, regularly shaped crystals from crystallisation, also:
After the remaining solution has cooled down, pour the excess liquid away (or filter it). Dry the crystals using a warm oven or by patting them with filter paper.