The preview tool allows you to get a preview of your message & its media before it is actually published. This is particularly useful when you publish a message that is a bit too long for Facebook or LinkedIn, as it allows you to know when the famous "see more..." message is displayed on LinkedIn or Facebook.
To do this, click on the small eye icon when you write your post to view it and adjust it to your liking.
You can preview your entire Instagram feed if you linked your Instagram accounts to Swello! The feed preview allows you to add your pending posts to your current feed and move them around as needed.