It usually breaks into two parts. The duty of loyalty means not putting your own interests ahead of the company's — no self-dealing, no quietly taking a business opportunity that belonged to the company, no undisclosed conflicts. The duty of care means making decisions with reasonable diligence: reading the material, asking sensible questions, not being reckless. Courts generally do not punish directors for decisions that turned out badly, only for decisions made carelessly or disloyally.
Founders acquire these duties the moment they take on co-owners, investors or a board seat, often without being told. They apply between business partners too. This matters most for anyone running several ventures: routing an opportunity that plainly belonged to one company into another you own more of is a textbook loyalty breach, however reasonable it felt. The remedy is disclosure and a documented decision by the people not conflicted — not silence and good intentions.
A concrete example: you are a director of a company with two outside investors, and a client approaches you about work squarely within that company's line of business. You are tempted to take it through your solo consultancy, where you keep 100%. Doing so quietly is a corporate opportunity problem. Disclosing it to the board, letting the disinterested directors decide whether the company wants it, and documenting the decision converts a potential lawsuit into a minuted item.