It's Important!
You've just done a google search, and are looking at the results page.  How will you decide which link is most relevant to what you searched for?  By reading the page title and description. 

Google Snippet Optimization Tool

Thanks to the Google SERP Snippet Optimization Tool you don't have to guess what your site description will look like when your page comes up as a google search result.  This tool by SEOmofo.com allows you to enter in an example of your page title and description to see exactly what fits and what doesn't.

Google's characters count

To be more precise the length of a title in google is 70 characters and the description length is 156 characters (including spaces) so make sure you use that space wisely! 

Writing your description

When you are writing your site description be sure to make it interesting and specific to what your site does and don't forget to include keywords that people will be searching for!

Helpful links about SEO Meta tags:

SEOmofo Snippet Optimization Tool
Length for Google, Bing, Ask.com and Yahoo by SageRock Digital Marketing Blog
Google Description Makeover by Official Google Webmaster Central Blog