SafeSearch filters explicit results. Google classifies pages automatically; site owners can also label content explicitly.
Labelling your own content
Use the rating meta tag with the value adult, or an equivalent X-Robots-Tag header, on pages that should be filtered. Applying it at page level is more accurate than at site level.
When classification is wrong
Sites in medical, art, fashion or education can be misclassified. Separate genuinely sensitive material onto its own URLs so that classification applies where it belongs, provide clear surrounding context, and file a report through the SafeSearch feedback form.
Alien Road recommends: isolate anything that could be misread onto its own section rather than mixing it with commercial pages. Misclassification tends to spread across a directory, and keeping the boundary sharp limits the damage to the part of the site where the content actually lives.