Press the exit edit button when you're done editing the page. After you've made your changes, you can either discard your changes, preview your changes, or publish them.
The preview changes option is handy if you've got a live website, but you'd like to see how the finished page looks before you publish the changes to your visitors. If you're yet to go live, it's best practise to publish the changes as you go.
You're also offered the option of 'version comments'. These aren't necessary, but may be useful later on. Each time you publish or preview a change, your change is saved as a 'version'. You can later revert back to this version, by clicking on the 'versions' button. If you've entered in 'version comments', this will make it easier for you to find the version of the page you're looking for.