Root rot is a condition found in houseplants with poor drainage. The roots of the plant rot as a result of over-watering. The excess water makes it very difficult for the roots to get the air that they need, causing them to decay. Severe over-watering usually results in the death of the plant. There is no effective treatment except to allow the plants to totally dry out and hope some roots survived.
To avoid root rot: always use containers with drip holes in the bottom; never allow a houseplant to sit in water for more than 10 minutes. For houseplants that require a great deal of water, keep the soil moist but never soggy. Most houseplants need to dry out a little before they require water. Houseplant soil should be light enough so that it drains quickly.