Imagine having an ability to see what is the hidden value of all your decisions. That's opportunity cost, the foregone profits, and possibilities you didn't pursue.
If you deal with limited resources, either time, mental or physical energy or money, you make choices. You have made a choice to run a business and that comes with its costs.
When you are choosing to run your own business, you invest your time and money and therefore can't use it anywhere else. You could have taken full-time corporate job and invest your savings, yet you didn't. Your business should compensate your for that choice.
Sometimes it isn't that easy to figure out what are your alternatives, but it is worth trying. Understanding them lets you make better choices, maybe even starting a business because possibility of high return and freedom from corporate rules is just worth it.
Opportunities and alternatives lie in an everyday life. Your budget allows for one more hire, do you choose product manager or sales representative? Do you focus on improving your product so you can sale it to more people for higher price, or do you expand your product line and maybe up-sell?
To complicate matters even more, your opportunities and options are not only costs, they have their own disadvantages and risks that should be weighted against each other.
Perhaps when hiring, your PM would be employee but SR would choose working as a contractor. Each option then comes with different risk profile, each business manager has different preferences, and they would make different choices.
All in all, opportunity costs are important for day to day tactical and strategic decision making. Bear in mind that in deciding, you should only consider future costs and future benefits. Never under any circumstances include sunk costs.