Add third-party page elements to your layout
Blogger lets users create new page elements for their layouts that they can then share with others via buttons on their site that say +Blogger or Add to Your Blog.
If you click on one of these buttons, you should be prompted to sign in to Blogger (if you aren't already), and then you can select one of your blogs to add the page element to. You'll also see the title, content and template for the new page element, which you can edit if you want. A logo and a "more info" link might also appear, to tell you more about where this page element comes from, who created it, and what it does.
When you click the Add widget button on this screen, you'll be adding code to your blog. It's important to make sure you either trust the source of this code, or have reviewed it to make sure it's something you're willing to have on your blog. Google claims no ownership or control over any content submitted, posted, or displayed by an author on or through Google services. In addition, blog authors who add code or third-party widgets do so at their own risk.
Once you've added the page element, it will appear on your blog's Layout, where you can edit, move, or delete it, just like the other parts of your page.