The idiom make hay while the sun shines draws from farming practices, where you should get as much done as possible at harvest time while the sun is shining, or your hay will spoil from getting wet.
It means that you should take advantage of the good times to get work done.
It means that you should take advantage of the good times to get work done.
We have a lot of customers at the moment, let's make hay while the sun shines!