What are they and what can they do for you?
Stakeholders are the people who stand to lose or gain in your topic.
For example, if you are writing about thyroid cancer, stakeholders would be victims of thyroid cancer, pharmaceutical companies that sponsor research about thyroid cancer, health insurance companies that pay for treatment of thyroid cancer, the government because it makes health policy to protect citizens who might get thyroid cancer, and probably others.
The advantage of knowing the stakeholders is that when you evaluate information a stakeholder produces, you already understand why they produced information, so you understand their bias, or perspective.
If you don't know who the stakeholders are, you can read some periodical articles (from a database), find stakeholders' names, and then use Google to determine if they have a web site.
Stakeholders in baby formula: should parents use it to feed their infants?
Google is what most people use to search the web. A random search on your topic can yield a million random web sites. If you think carefully about who the stakeholders are in your topic, you can use Google to find the web presence of organizations such as companies, government agencies, think tanks, consumer groups--in short, people who have a reason to convince you of their point of view.