"Make hay while the sun shines" means that one must take advantage of doing something while the conditions are favorable to do it. Procrastination sometimes results in the situation changing, making it harder to do what is necessary or making it impossible to do the task at all. One must avail himself of an opportunity when it presents itself lest that opportunity get away from him.
Working with hay is difficult in wet conditions, so sunny weather proves advantageous for this particular chore. If the chore is not completed when the weather is good, bad weather right around the corner prohibits the chore from getting done. Several other sayings exist with similar meanings. For instance, "opportunity only knocks once" indicates that when a chance to do something is passed, it often does not return. "Carpe diem," a Latin phrase meaning "seize the day," is another similar saying. "Do not put off until tomorrow what you can do today" is still another, as is "Strike while the iron is hot." The original phrase is used in many situations. For instance, when the stock market is doing well, investors say they need to "make hay while the sun shines." This means that they need to make money while the market is good because sooner or later it reverses.