The end user is the individual who ultimately uses or is intended to ultimately use a product or a service.
Here are two examples to help you understand:
Intercom is a SaaS company that offers a customer support solution. When a company implements Intercom on its website, customers can use Intercom to ask questions and employees to answer them. Customers and employees will be considered end users of Intercom, and the company its user.
Slack is a SaaS company that offers a B2B messaging platform. The companies that uses Slack are its users and the companies' employees (the ones who will ultimately use the platform to talk and collaborate on internal projects) are considered its end users.