Melanoma In Situ is being diagnosed with increasing frequency. Skill with dermoscopy is one reason, increaing awareness of the dangers of atypical dark lesions another, trends in dermatopathology a third. There are a number of other theories.
Simply MIS is a Stage 0 lesion. The link above is accurate and helpful. Strictly speaking MIS is not cancer -- yet. And no one knows whether these really progress to Stage I -- but the risks of waiting (for most people) outweigh the benefits. The cure rate for MIS is 100% at ten years.