ATIS - Automated Terminal Information Service.
A continuous loop of weather and airfield data transmitted on a frequency which the pilot dials up and listens to when inbound / outbound. Saves ATC having to give everybody the weather individually.
Each broadcast starts with an Information letter - first one of the day normally Alpha. When the information changes, the letter changes. This way ATC know which information the aircraft has and can give them any relevant changes if necessary.