For the most part, only one person (the webmaster) will be making changes to the website. a page or file at time. However, conflicts do arise where a page might be getting edited by multiple people. Also if someone edited a page in the past but didn't check it back in, it will still be "checked out" to them and will be "locked." This is indicated when nothing happens when you click on GEAR>Edit page to try to edit the page (or if a strange message appears, see below).
If the above page appears, this page is currently checked out to someone else and they may be in the process or modifying it. Often it is because they forgot to check it back in. To check to see if this is the case & who the page is currently checked out to, click the GEAR>Show Ribbon. If someone has the file checked out, there will be a yellow banner under the ribbon noting this. Until they check it back in, you will not be able to edit it.
If you need to make edits to a page that is checked out by someone else, there is an option to override the person's checkout. Before doing this, please check with the person who has it checked out and make sure they are not in the middle of editing the page. If you override their checkout, their changes will be lost. To override a checkout, click on the "Page" option in the upper left corner and select "Override Check Out". This will checkout the page to you and you can begin editing the page.