Pinafore eroticism is when people find things that look like a pinafore dress, which is an apron-like dress that's worn over other clothing, to be sexy or arousing.
Imagine you have a really pretty dress that covers the front of your body, like a bib, and it ties at the back. Some people like to wear or see others wearing similar dresses because they find it attractive.
It's important to remember that everyone has their own preferences and what someone finds erotic may not be the same as what someone else finds erotic. It's also important to make sure that all sexual activities are consensual and safe.