If you have a page opened and have not yet closed it, the page remains checked out to you. When another user tries to open the page, they are informed that you are currently editing the page and they will be given the opportunity to override you. The warning message does state that all changes will be lost if they override the editor. If you have not saved your changes and someone, like an Admin, overrides you, your changes will be lost.
It is recommended to check with someone who has a page checked out in "Quick edit" mode before attempting to override. It is also important to remember to save and major or important changes regularly.