What it probably means is that C++ grammar is syntactically ambiguous, that you can write down some code that could mean different things, depending on context. (The grammar is a description of the syntax of a language. It's what determines that a + b is an addition operation, involving variables a and b.)
For example, foo bar(int(x));, as written, could be a declaration of a variable called bar, of type foo, with int(x) being an initializer. It could also be a declaration of a function called bar, taking an int, and returning a foo. This is defined within the language, but not as a part of the grammar.
The grammar of a programming language is important. First, it's a way to understand the language, and, second, it's part of compiling that can be made fast. Therefore, C++ compilers are harder to write and slower to use than if C++ had an unambiguous grammar. Also, it's easier to make certain classes of bugs, although a good compiler will provide enough clues.