Headings are great to grab your attention. This is a secondary heading to the main blog title.
Sometimes you'll want to have a few sentences or paragraph to hook people in to the main text content. This may appear at the top of the post or as a pull out of something particularly important lower down in the page. Just like you see in magazines.
Then comes the main body content. This will usually be a larger body of text and you may even want to include text links (Skip to around 2:27 on video below). These could link to a page on your website, or another website entirely. The general rule is that if it links within your own website just have a direct link. If it's going to an external website chose the 'open in new window' option.
Take a look at the video for step by step instructions of how to do all of the above. You may want to check out Squarespace's help centre where you can view tonnes of instructions and videos showing you how to get the most out of your website.