What is contextual selector?

Contextual selector is a selector that addresses specific occurrence of an element. It is a string of individual selectors separated by white space, a search pattern, where only the last element in the pattern is addressed providing it matches the specified context.

TD P CODE {color: red}

The element CODE will be displayed in red but only if it occurs in the context of the element P which must occur in the context of the element TD.

TD P CODE, H1 EM {color: red}

The element CODE will be displayed in red as described above AND the element EM will also be red but only if it occurs in the context of H1

P .footnote {color: red}

Any element with CLASS footnote will be red but only if it occurs in the context of P

P .footnote [lang]{color: red}

Any element with attribute LANG will be red but only if it is classed as "footnote" and occurs in the context of P