It helps you to get more traffic from search engines like google,yahoo,msn,etc and increase your search engine page rank value.These tags not only describes your site but also informs the search engines what your site is all about.
Meta tags allows search engines to index your web pages more accurately.Meta tags communicate with the search engines and tells more information about your site and make it index correctly and accurately.
You have to add two meta tags to the head section of the template.
First is the Meta Description which describes your site and another.
Second is Meta Keywords which tells about your site keywords.
Here is how to add meta tags to your blogger(blogspot) blog:-
1.Sign in to your blogger dashboard>click the 'Layout' button

2.Click on the 'Edit HTML' tab

Here is the code you have to add
<meta content='DESCRIPTION HERE' name='description'/>
<meta content='KEYWORDS HERE' name='keywords'/>
<meta content='AUTHOR NAME HERE' name='author'/>
But replace this RED coloured words with your site data.
For example:-
DESCRIPTION HERE : Write your blog description
KEYWORDS : Write the keywords of your blog
AUTHOR NAME : Write the author's name(Your name)
3.Add the above code just after this
<b:include data='blog' name='all-head-content'/>
Your edited HTML code will look like this
4.Don't forget to add description and keywords of your site.5.Now save the changes.