what is the equivalent for category in wordpress in drupal websites?
Everything you mention in your question is a Taxonomy, a way to sort your content into meaningful groups. This mostly helps to improve site navigation, discoverability and consequently clarifies what the website it about (boosting a keyword ranking might be too strong of a claim). 

In WordPress, it's custom to name the main Taxonomy Vocabulary as Categories (they can be hierarchical) and to have additional Taxonomy Vocabulary tags that are optional and sort the content into categories that aren't necessarily closely related to the website main topics. 

In Drupal, Taxonomy module allows you to create your own Vocabularies and use them as you see fit. You get the Vocabulary named Tags out of the box, but don't let that name confuse you. You can use Tags as your main Categories Vocabulary. You can also rename the label "Tags" into "Categories" (machine name will remain tags but that isn't user-facing). 

Or simply don't use Tags Vocabulary anywhere, delete the field from Articles content type that references it. Create all new Vocabularies, name them as you like (Categories, and Tags, with different machine name) and use them the same way as you did in WordPress.

There are no technical differences between terms coming from different Vocabularies. A term from Categories, and a term from Tags is linked to an article in the same way technically. 

The SEO value of using Taxonomy is in how smart you are about categorizing your content, knowing your visitors and offering them what they expect/want/value.