Before jumping into the concept, you must understand the core difference between Roles and Capabilities. These are the deeds that a user can commence. When somebody changes the appearance of the website by appearance section in the back-end panel, this can be related to capabilities.
A user having a set of competencies such as editing a post and adding a post is an example of roles. So, basically, a WordPress user who possesses such a set of capabilities is assigned the role of "Admin".
So, in simple Admin is someone who's got the license of managing the entire website. However, you can also use the best nopcommerce custom plugin from https://nop-tech.com/ to make your eCommerce website.
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The default setting for WordPress contains 6 roles and around 57 capabilities. Every role contains different-different combinations of capabilities and it is awarded to the users who have the rights and honors. And it is not recommended to modify any default setting, but many times changes work in favor.
Roles and capabilities are like heads and tales of a coin. A website administrator can always start off by creating a set of new capabilities. For example, if you own an eCommerce website. Then it's obvious to have a marketing team.
Then the marketing team doesn't need the admittance to the post of the website, however, they might need access to trending search results, advertising statistics, etc.
So, as a programmer, you will be responsible for assigning explicit roles to the marketing team. Naturally, they should not be assigned any role regarding adding and editing a post on the website as it did not fall under their niches.