function field will get automatically call without any event or button.
If store=True will store the value of the field in database.
Once stored then the functional fields function will not be executed again. If 'store'=False, then every time(any change in the record) the functional field will be executed
from this link you can get more detail: [https://doc.openerp.com/trunk/server/03_module_dev_02/#functional-fields][1]
Hope this help.