The month is closed by raising the security level. Many users operate on level 3 with level 9 being the highest security level. You may have intermediate users who can post journals etc. at another point such as level 5. In which case you may wish to change the access level to 5 for the initial (preliminary) close, before performing a final close by changing the access to level 9.
Masters > Periods > Select the period > Change to Access Level 9.