The conversion of precipitate into colloidal solution by adding a suitable peptizing agent, is called peptization.
The peptizing agent is a stabilizing agent and they are basically electrolytes.
For example, when the ferric chloride solution is added to the precipitate of ferric hydroxide, the hydroxide precipitate transfer to the sol by absorbing ferric ions.
In simple words, Fe3+ is absorbed on Fe(OH)3 and splits into colloids.
Ions either (+ve or –ve) of the peptizing agent (electrolyte)are adsorbed on the particles of the precipitate. They repel and hit each other breaking the particles of the precipitate into colloidal size.