The visualization presents monthly global temperature anomalies. This visualization is updated roughly two weeks after the end of each month.
Temperature anomalies are deviations from a long term global avergage. In this case the period 1951-1980 is used to define the baseline for the anomaly. These temperatures are based on the GISS Surface Temperature Analysis (GISTEMP v4), an estimate of global surface temperature change. The data file used to create this visualization is publically accessible here.