Periodontal Cleaning (Deep Cleaning) is required for patients who have heavy deposits & haven't been to a dentist/hygienist for a long time and also for the treatment of gum disease. This requires a deeper scaling which involves removal of plaque/calculus deposited on the root surfaces below the gum line.
Your dentist may discuss the option of giving you a local anaesthetic, so that it does not feel too sensitive and uncomfortable. It takes longer than a regular scale and polish. Your mouth might be treated in sections, at more than one visit, if required.