The description tag is what the visitor will read in the search results when deciding whether or not to click on your listing.
Your description tag is what people will read in the search results before deciding whether or not to click on your link. Think of it as a two line classified ad and maximize every word.
Does it compel users, and is it a good description of what users will get when they arrive?
Like the title, it is important that the description is relevant to the page content.