A literal expression is simply anything that COMPILES to a literal. All numbers, all strings, all defined constants (including constant arrays) and some expressions are compiled to a literal. In the glossary you can find what compiles to a literal, but we list them here too:
| ' | <name> |
| ['] | <name> |
| CHAR | <char> |
| [CHAR] | <char> |
| C" | <string>" |
| " | <string>" |