As a business owner your time is often overwhelmed with the day-to-day work required to run your business. You knew you needed to form a limited liability company or a company with your fellow owners and you did that. So what is this thing you heard about needing to plan past the formation of your business? What is a buy-sell agreement?
The simple answer is a buy-sell agreement is an agreement between the owners of a business that controls what is going to happen if one of the owners leaves the business. Leaving the business might be by choice (wanting to sell your share of the business to someone else) or by circumstance (one of the owners suddenly passes away).
When you and your fellow owners started your business you went in with the intent that you were going to work with those individuals. You did not go in with the intent that they could sell their share to some stranger or pass their share of the business to their children or grandchildren. A buy-sell agreement helps you take control over those types of situations.