It's an annotation to overcome compiler warnings regarding unchecked generic operations (no limitations), such as characters. It indicates that the programmer didn't want to be notified about these which he is previously knowledgeable of when compiling a particular piece of code.
To read more on this particular annotation, click here:
Additionally, Oracle presents some tutorial documentation on the acceptance of annotations, click here.
As they mention,
"The 'unchecked' warning can occur when interfacing with legacy code written before the advent of generics (discussed in the lesson titled Generics)."