A constant made available to a process, by inclusion in the executable text. Most modern systems do not allow texts to modify themselves during execution, so literals are indeed constant; their value is written at compile-time and is read-only at run time.
In contrast, values placed in variables or files and accessed by the process via a symbolic name, can be changed during execution. This may be an asset. For example, messages can be given in a choice of languages by placing the translation in a file.
Literals are used when such modification is not desired. The name of the file mentioned above (not its content), or a physical constant such as 3.14159, might be coded as a literal. Literals can be accessed quickly, a potential advantage of their use.