In short, the Reading & Writing section tests your ability to read, comprehend, and analyze passages, and your ability to adhere to the standard conventions of grammar and sentence structure. While the old paper-based test separated these into two different sections (Reading, and Writing & Language), the new digital SAT combines them into a single section (Reading & Writing).
While this was covered earlier, it's worth mentioning again in case you skipped to here or forgot. The Reading & Writing consists of two modules, each of which is 35 minutes long and has 27 questions. This averages out to 78 seconds per question, but don't concern yourself with this statistic, since different questions will take vastly different amounts of time.
Each question has its own short passage (up to about 150 words), with 4 multiple choice options. This contrasts with the old paper SAT, where each Reading section consisted of several long passages, each followed by numerous questions about that passage.
In the Reading & Writing section, the questions are grouped together by question type. For example, you'll get several "words in context" (basically vocab) questions in a row, followed by several "text structure and purpose" questions, and so forth. The various types of questions on the SAT will always fall in the same order, so you can have an idea of where to expect various types of questions.
The College Board has their own way they categorize the questions. They identify 4 "domains" that Reading & Writing questions fall into. Then, within each domain, they identify multiple "skills" that questions in that domain might fall into. While that's as far as they go in their official categorization, there are further subtypes of questions that can be identified. We'll cover all these subtypes, how to identify them, and what strategies to employ for each type.
Below are the 4 Reading & Writing domains that College Board uses to categorize questions, along with College Board's description of each domain. The order shown below is the order in which they appear on the test. Note that College Board's definition is often an oversimplification of what the domain actually tests. It's more important to understand each of the different subtypes of questions than to think about what "domain" it falls into.
As mentioned previously, College Board identifies multiple "skills" fall under each domain. Below are the skills College Board identifies, but keep in mind that many of these skills also have subtypes that can be identified, which we will cover later. The order shown below is the order in which these skills appear on the test.
We'll get into more detail later about what each of these skills actually tests, and the subtypes of questions that fall under them.