The "Other" portion of your storage contains miscellaneous data such as browser cache, browser history, stray documents, messages, etc. The simplest way to free up space is to launch the System Information app through your applications folder. Once System Information is open, press the "Command" and "U" keys at the same time. A storage management page will appear. From here, you will be able to view all of the files that are hidden within the "Other" category. For example, you can click on the "Documents" tab and delete what you don't need.