A cost overhead is the amount of money added on top of your employees salary. Productive calculates overheads based on your average facility costs (office space, software, travel expenses etc..) and on the amount of time you spend on internal projects (like Sales, Finance, HR etc..)
Click on your avatar, go to Settings,
and select Cost overhead.
Click on the Edit cost link, put down the amount you spend monthly on facility costs and Productive will calculate the rest.