For such a long time I didn't read this book.
In my opinion, the relational model is a mathematical model to describe the theory behind the RDBMS( Relational Database System), so it can represent some items or objects more clearly than sentences.
Relation is just a table. And it has some rows and colomns.
In a relation, there are some colomns, and a named colomns is called an attribute.
Each attribute must have some constraints to its value. So, the domain is the set of allowable values for one or more attributes.
It is a row of a relation.(or in a table)
A collection of normalized relations with distinct relation names. Normalized is explained as “appropriately structured”. Some….Kind of ? Maybe I can understand but can't tell.