A VO is a virtual organisation, a method of grouping people by mutual interests or projects using digital X.509 certificates. A VOMS is a virtual organisation membership service. VOMSes is the plural of VOMS.
This page allows you to set up VOs. The first tab 'VOMSES Directory' allows you to choose the directory which contains files with details about virtual organisations. The second tab 'VOMS Server Certificates' allows you to choose a directory which contains the appropriate certificates (in .pem format).