What does it mean for a solution to be ionized?

Ionization occurs when atoms or molecules gain or lose electrons. They, therefore, become electrically charged ions. Electrons are negatively charged while protons are positively charged. If there is an equal number of protons and electrons, the atom or molecule is neutral and not ionized. In a solution, molecules can disassociate to release positively charged and negatively charged ions. For example if an acid like hydrochloric acid is added to water, the hydrogen ions (H+) will be released in the solution making a solution of charged particles. The solution is ionized.

When a solution is said to be ionized, it means that the molecules of the solution which were formerly neutral have been converted to charged particles called ions.