Ionization is the process by which atoms and molecules acquire a positive charge by gaining electrons or acquire a negative charge by losing electrons.
In an aqueous solution, the molecules of an electrolyte undergo spontaneous dissociation to form positive and negative ions which ionize into ions.
If the percentage of dissociation of the atoms or molecules into ions is 100% that means it is completely ionized in the solution.
Some acids and bases ionize almost completely in solution and are known as Strong acids and strong bases.