The first line here pulls the user out of the database using the submitted email address. The next line is common in idiomatic Ruby:

This uses && (logical and) to determine if the resulting user is valid. Taking into account that any object other than nil and false itself is true in a boolean context (Section 4.2.3), the possibilities appear as in Table 8.2. We see from Table 8.2 that the if statement is true only if a user with the given email both exists in the database and has the given password, exactly as required.