Codd's relational database model was specifically designed to represent tables and the 1:N relationships between them. It can also represent 1:1 relationships by treating them as a special type of 1:N relationship (i.e., as a 1:N relationship with an extra condition/index that prevents multiple children.)
The relational database model works great for 1:1 and 1:N relationships. However, as surprising as this may seem, the relational database model cannot (directly) store M:N relationships.
What is a M:N relationship? The relationship between students and courses is M:N. The spousal relationship between males and females is 1:1, but the dating relationship between them is M:N. The relationship between customers and products is M:N. The relationship between customers and the store locations where they shop is M:N.
Now, wait a minute! Keeping track of students, courses, dating, and customer shopping habits seems pretty darn important. Can't the relational database model do this for us?
No, not directly. (And Codd is considered a genius?)
But, where there's a problem, there's a fix. The relational database model can represent a M:N relationship by breaking it into a couple of 1:N relationships (and a new table called a bridge/composite entity).
The M:N relationship between students and courses is stored by creating a new "registration" table. There is a 1:N relationship between students and the registration table. And, there is a 1:N relationship between courses and the registration table. The records in the registration table are children of the students and the courses. The registration table has two foreign keys: one to the student (mother) and another to the course (father). The registration table serves as the bridge between the student and course tables.