Stand development is a principle that involves initial agitation (longer than normal, some recommend up to 2 minutes), then the tank sits for a much longer than normal time. (not for open tray schemes) Semi stand is similar at the beginning, but introduces agitation at intervals of 3 - 5 minutes, to ensure uniform development in even (eg - sky) areas.
I have left a lot out here - use the search engine for this forum, and enter the two terms "stand" and "semi-stand". you'll find lots to read, opinion and fact both. People have widely varied experiences with this method.