The meta description as the name tells is a short description of your posts, the meta description is supported by almost all major search engines, e.g.: Google , Yahoo , MSN....
If you google it probably you will found a code what is looking like this one:
<meta name='DESCRIPTION' content='A few words about the content of the page'/>
This is the basic code of a meta description but this is usually used for webpages not for blogs because this code will give the same description to every post of your blog. If we want to have a well SEO optimized blog we need to use different descriptions for each post. To use different description for every post complete the following steps:


1. Open the Layout panel and go to Edit HTML
2. Save your template by clicking on the Download full template button (to backup if something goes wrong)
3. Find the following code:
<b:include data='blog' name='all-head-content'/>
4. Add this code after the above code:
<b:if cond='data:blog.url == "http://articles-city.blogspot.com/2010/12/journey-by-lee.html"'>
<meta name='DESCRIPTION' content='SEO Tips Blogger , my blog about Articles jokes .'/></b:if>
5. Change the red marked url with the full url of your post.
6. Change the green marked text with the description of your post
7. Create a code like this for each post.

8. Save your template.

NOTE: Don`t use more than 150 characters in your description.
Add the code for a post and put the code for the next post directly after it. To add description to the main page change the red marked url with the homepage of your blog: e.g: http://articles-city.blogspot.com/