Warm surfaces radiate more in the longwave range.
Low values of OLR are typically due to clouds in the atmosphere. The OLR animation shows two maps. The top map is the climatological average for that month, ie the April map is the average of all the Aprill's from 1975 to 1987 when this data was available. The lower map shows the anomalies for each particular month and year. This is the difference between the long term average and the actual data for a single month.