It is the total rainfall for an average year, expressed in millimeters. A millimeter is one liter per square meter, or ten thousand liters per hectare.
The value shown is a climatic normal for the period 1981-2010: it is not what it rained last year or what it will rain next year, but the thirty-year average. It is used to know what regime the property is in, not to plan a campaign.
The 600 mm threshold
Below approximately 600 mm per year, rainfed farming agriculture—without irrigation—is no longer reliable in Argentina. It is not that it cannot be planted: it is that the probability of losing the campaign due to lack of water becomes high, and the farming system becomes livestock or mixed, or needs irrigation.
The annual average hides the distribution. Eight hundred millimeters distributed throughout the year are not the same as eight hundred concentrated in four months, and for the crop the difference is decisive.