It's a briefing format designed to highlight the significant threats during an arrival or departure, and to communicate how those threats will be managed. It consists of several modules:
C - ChartA number of mandatory items, including SID/Transition, STAR/Approach, chart numbers, navaid tuning, final approach track, airport elevation, DA/MDA/RVR, missed approach track & altitude, etc.
T - TerrainRelevant MSA, location of any significant terrain.
W - WeatherSignificant weather, e.g. strong winds, windshear, low visibility, etc
O - OperationalSignificant operational requirements, e.g. non-normal procedures, NOTAMs, landing performance, fuel, etc.
PlusOther threats not covered above.