What do we mean by the term "resource" and what are the different resources?

A definition from Wikipedia on this:
"French = 'la ressource', or Latin 'resurgere' = to spring forth, source.
A resource is a tool, a condition or a characteristic or property to pursue goals, to manage requirements, to perform specific actions or to let a process run in a goal-oriented manner".

Thus, in project management there are many different resources that are needed to achieve a given project goal, such as:
- Operating equipment (machines, installations, ...)
- Objects (test equipment, climatic chambers, ...)
- Material (circuit boards, electrical and mechanical components, ...)
- test environments (EMC, mechanical stress, temperature, chemical resistance, IP protection, ...)
- and last but not least "people" with their different skills.

In addition, there are also so-called "generic resources" in project management.
With these special imaginary resources and their corresponding skills, a first project schedule can be developed. These generic resources are then replaced with real resources during the ongoing project. Due to the fact that the required resource is not always available, the first shifts in the plan occur automatically.
A resource can therefore be anything that is needed for a specific purpose within the scope of the development. If only one of these many different "puzzle pieces" is missing, this will automatically lead to delays in the project.

In the context of project planning, one can usually rely on a material resource such as a PC, a test environment or software. It is assumed that it will work as planned. 
With the particular required "human resources" with their special characteristics and skills (technical expertise, experience, ...) planning is not so easy.
The time a person invests in a project does not always automatically equate to the performance she or he delivers during this time. The individual operational capability of the human resource can be seen as a weakness. At the same time, however, it is also its greatest strength.

The human being is the most important resource

In the current project, we first talk about the departments, the employees or the team. 
In planning, we lose this "human" and instead speak of the resource.
Thus, each project manager can use personal resources from an existing portfolio for his projects (of course only if available). These resources differ, as already mentioned, in their knowledge, skills and experience. This is also called the special know-how, or the special knowledge of an employee.

Physical resources such as objects, rooms, materials and so on are only tools in the ongoing project. However, people are absolutely essential for the successful realization of development projects. It is therefore not uncommon to speak of the human factor, which is the most important influencing factor in key situations. 
Since each specialist can be used for different activities within a project based on her or his own individual skills and strengths, people are therefore objectified as a resource in the planning phase. However, this should not decrease the value of an employee, or reduce it only to this specifically needed characteristic!
Thus, beyond the actual resource planning, one must also consider the individual employee; Because all people are different, have their own interests, strengths and weaknesses.
That means that beyond planning, one must also consider personal things such as: 
- In which environment/situation does the employee feel comfortable?
- Who has which character traits?
- Does the chemistry in the team fit together?

Conclusion:

Yes, people are the most important "resource", not only in development projects, but in the complete company!
Nevertheless, you should not use the term "resource" in discussions with employees in order not to give the person the feeling that she or he is only a tool. Resource always sounds as if you can use it without hesitation.
Much better here are terms such as colleague, team member, ...
No matter how technical the development projects are, the "human element" should not be lost.
And who knows, if you take the time and get to know the people personally and also appreciate them, you may also recognize other unknown skills, and can use this unknown potential in future projects.

For more information on how resource management is implemented at MOBA, read the report:
   The MOBA Project Management - Part 11 "Resource management".
   https://mobacommunity.com/blogs/entry/The-MOBA-Project-Management-Part-11