Sometimes you might need to edit multiple posts, perhaps to tag them with a new category or a keyword, or you might need to change authors, turn on/off comments, etc.
Rather than editing each post one-by-one, you can go to your Dashboard and click on Pages/All Pages. Select the posts you want to edit by clicking on the square selection box to the left of the title, then click on the Bulk Actions/Edit drop down menu item; click on Apply.
You now have the option to add or remove tags or categories, as well as change the author, change commenting options, or change the status of the posts, amongst other things. Hit the Update button to apply the changes.
The same process can be done to modify many pages, though with a few less options.