A literal, as opposed to a variable, is a constant value of a given data type.
Literals can be assigned to a variable of the same type :
sName = 'Olivier'
nAge = 39
bIsSmart = true
Literals can also be used in expressions :
nTomorrow = Today() + 1
sFullName = 'Olivier' + sFamilyName
In all cases, note how the Code Editor uses syntax highlighting to identify string literals in blue, integer literals in red and boolean literals in grey.