Albert Barnes Commentary Matthew 20:12

"saying, These last have spent [but] one hour, and thou hast made them equal unto us, who have borne the burden of the day and the scorching heat." — Matthew 20:12 (ASV)

The burden and the heat of the day. The burden means the heavy labour, the severe toil. We have continued at that toil, in the heat of the day. The others had worked only a little while, and that in the cool of the evening, and when it was far more pleasant and much less fatiguing.