Meta tags are useful for search engine optimization purposes: they associate key words to a website. For instance, if your blog is about cooking, you may want to add meta tags such as "recipes, cooking, bakery, cup cakes" to your blog. This will help google, yahoo, bing, etc to recognize the topics of interest on your website.
To add a meta tag to all posts of a blog hosted on Blogger, go to "
template > modify html code > handleand edit the html source of the blog to add your meta tag in the html header.