The only person who has ever written the word 'benevolent' before 'skin cancer' is Hemingway, in the Old Man and the Sea: "The brown blotches of the benevolent skin cancer the sun brings from its reflection on the tropic sea were on his cheeks." Cancer is never usually described as benevolent. Some critics believe Hemingway deliberately used the wrong word, others view it as an uncorrected error. The usual term is 'benign', meaning not harmful. This is the opposite of 'malign'.

... and btw, your question is fine apart from the unnecessary articles. Diseases don't take articles. It should be ' I know what skin cancer is, but I don't quite understand what benevolent skin cancer is.'

Did you mean benign skin cancer? Cancers can be malignant (the dangerous ones) or benign (the not so dangerous ones).