Your question: What does it do and how does the code sample work?

Code translation:

if !ErrorHasOccured() || HandleError();

that code is equivalent to:

if (ErrorHasOccured()) HandleError();

How does it work? : In C trigraph,

??! is equal to |. So, ??!??! means ||

P.S: This type of code was used when some of the keyboard did not have | key.