Semi-formal wedding attire sits between cocktail and black tie on the spectrum. It is the most common dress code on modern wedding invitations and, usefully, the one with the widest latitude for interpretation. The core idea: put effort in, but do not overreach. You are dressing for a thoughtful event, not a gala.
For women, semi-formal means a midi or tea-length dress, a jumpsuit, or a dressy separates combination (skirt and a fine top). For men, it means a suit with a tie, or a sport coat and trousers with a tie. Nobody is in a ballgown or a tuxedo; nobody is in jeans.