To take an opportunity to do something when the time and conditions are near perfect or available.

From agriculture, when farmers gather the long grass growth of summer to dry it to produce hay as winter feed for animals; The best time to cut and dry the hay is during sunny days.

To take advantage of a brief opportunity while it is still there. To not waste time.

The saying comes from farming communities. There would often be only a short time in which the hay would be ready to cut- reap too soon, and the hay would be too green. Reap too late, and you run the risk of rain ruining it all. Still very good advice.

To do something while you have the time to do it.