Deck-O Seal or Mastic is the caulking that goes around the circumference of your pool between the pool decking and the coping (brick or stone). The sealant functions as an expansion joint, expanding and contracting in warm and cold conditions. Without this sealant, water could leak into the gap between the pool deck and the wall of the house. There are two types of mastic: white and black. Both types can be used for sealing around pools, but black mastic is more resistant to heat and sunlight than white mastic. If you plan to leave your pool unattended for an extended period of time, it may need to be sealed with black mastic to prevent damage from occurring due to heat expansion and contraction. Pool seals should always be applied by a professional contractor or pool service company. The last thing you want is to spend money on a new pool only to have poor workmanship cause further problems down the road. If you are doing some DIY work on your pool, make sure not to get any of the mastic on your skin or on any other surface inside your home. Some of it will likely get on your hands even if you're careful, so take precautionary measures to keep yourself safe while working on your pool. The best way to ensure good quality work from a professional contractor is to hire them through a referral from someone you trust.