By default, every Account doesn't have any Group assigned to it. You may want to not use it, but you would be missing out on a feature that can be pretty useful.
After reading this article, you will know what Groups are and how they can be used.
Let's get started
Where can you find the information about a Group?
The Group can be found when opening an Account record, in the top-left section:

What are Groups?
Groups are a type of tags that allow you to classify your Accounts in terms of different criterias that you find relevant for your organisation.
Generally, they are used to distinguish the industry, but can be used for something else completely, depending on how you want to use them.
What are they used for?
Groups can be used mostly for marketing purpose.
You can easily filter Accounts that share a same Group and target them with a marketing campaign.
You can also quickly see an important information when opening an Account record.