What is that thing between CSS "selectors" called?

As identified by Tom Haws, the operators between the simple selectors are called combinators. In CSS2 there were only three: +, >, and the space combinator.

• The space is the combinator used in a CSS descendent selector.
• > is the combinator used in a CSS child selector.
• + is the combinator used in a CSS adjacent sibling selector.

In each case, the 'selector' is the full combination of the simple selectors and the combinators.

The range of valid combinators expanded, once CSS Selectors Level 3 was standardized, to include the ~ or "subsequent-sibling" combinator.

According to https://www.w3.org/TR/selectors-3/#combinators they are called "combinators".

1. (space character) = descendant combinator
2. > (angle bracket or greater-than sign) = child combinator
3. + (plus mark) = adjacent sibling combinator
4. ~ (tilde) = general sibling combinator

The characters or whitespace between tag names are called combinators, see for example General Sibling combinator. These are > and + in your example.

The tags in your example are called simple selector in CSS2 and CSS3. If you would have a b c that would be called sequence of simple selectors in CSS3 but simple selector in CSS2. The term simple selector does only refer to one element name in CSS3 such as a in a b c.

Or as the section Selector syntax states

A selector is a chain of one or more sequences of simple selectors separated by combinators.

a b c, d e f is called the group of selectors where the group members are the selectors a b c and d e f. a b c is a selector, or sequence of simple selectors, composed of the simple selectors a, b, c combined by the combinator whitespace. The last sentence is only valid for CSS3.