SST (Sea Surface Temperature)
SST is the temperature of the ocean's surface, measured by satellite rather than by a thermometer in the water. Infrared sensors read the top few micrometers of the surface; microwave sensors read a slightly different, marginally deeper layer. See why doesn't my boat's gauge match the chart for how that surface reading compares to what a transducer reads below.
On Chlorofishy, SST is shown as either a raw satellite pass or a gap-filled composite. Full explanation: raw pass vs. composite SST.