Search Engine description is simply how search engines like google, yahoo, bing and other search engines describe any of you blog post. Its Just like adding Post Tags in wordpress..
Adding A Search engine description simply tells search engines how you want your post to be described otherwise, the search engine will automatically get the description from your post title which in real sense might not fully describe your blog post and this is why the manual addin has to be employed.
To Add This Feature In Your Blog, Follow the Steps Below:
** Goto Your Blogger Dachboard
** Locate The Blog You Want to add Description To.
** Goto Settings
** Locate Search Preferences
** Click The "edit" Next to "Description" to Open a Text Box (as below) Select "Yes" and Enter Your blog description text. Then click "Save changes" to save the Changes.
After Enabling This Option, You will then have a search engine description option in every post (as in the image below).. It is optional meaning that you can skip it whenever adding a post but its adviceable to add it for search engines to describe your blog post in the right manner without establishing one for you based on the post content..