“Make hay when the sun shines” is a saying to describe a situation where you might be in a great position and as such you should try and maximise any benefits you can get while it lasts. In business, this may apply when there is a boom in a particular sector, there are government grants or investors for particular business areas.
For example…
Take app-developers for smartphones. They are definitely making hay while the sun is shining! Smartphones and tablet sales are consistently rising and developers who can make apps are reaping financial benefits from an increased demand for making apps to deliver common services from banking and groceries to entertainment like games and education.