My partners groin is swollown what could this be?

Hernia? Nodes? Groin swelling can be due to a hernia (bulge of intestine through a weakness in the muscular support), could be swollen lymph nodes from infection, tumor, other. See md.
Need more info. What do you mean by "groin"? Is this accompanied by other symptoms (fever, enlarged lymph nodes, ulcers, discharge, etc.)? Is your partner male or female? How long has it been swollen? Is the area involved red, hot, tender, discolored? Reframe the question and will try to be of more help.