What is a malformed token?
The basic syntactic units in any textual programming language are tokens, which are, if you wish, the words in the programming language. The compiler parses these tokens to build sentences and so forth.

A malformed token is a string of characters that is not a valid word for the programming language. 

It is not a variable that doesn't match the naming rules – not all tokens are variables. It is not an operator that is unknown – sometimes operators can be defined by the programmer. It is one of the smallest pieces of syntax that will never correspond to something sensible.