'stakeholders' basically means all the people who have some interest in, involved in or affected by something.
It is a term most commonly used for projects (both business and non-business projects). In a business project for example, the 'stakeholders' would be the people in the project team, management, owners, staff of the company and the customers/end users who will be affected by the project. It is common to have representatives of all the 'stakeholders' in project meetings to avoid future problems.
It is often confused with 'shareholder'. But 'shareholders' only means the people who own a part of a company (they own shares/stock in the company).
In a project to build a new road, the 'stakeholders' would include local residents, local and central government, the company building the road, local companies, the project team.
To learn more vocabulary connected to projects, you can do a free online exercise on project related vocabulary.