Closing forms
There are three ways in which a form can be closed; automatically by the number of submissions, automatically by the close date passing or manually by you selecting to close the form. The first two are defined when you are adding the form and the latter is by using the close icon 
on the form list.
You may have noticed that some forms do not have the manual close option. This is because they have been automatically closed via one of the former two methods and therefor cannot be manually opened or closed.
The icon works as a toggle, so you can select it to close the form as well as open a (manually) closed form.