The description tag is one of the meta data. This tag has no effect on the ranking of your site in search engines. The search engine will read the description tag of the website and display it under the listing. It's recommended to use descriptive description about the services or products you provide to make the user click on your listing in the search results. The better the description the more visitors will click on it, off course your site also needs to rank well in search engines. You do not need to spam this tag full of keywords as it will not help and has only a negative effect.
This is How Your Description Tag Should Look Like
If you're using these guidelines we described above to optimize your website it will show an improvement on visitors and postion in the search engines.
Make sure that the keywords you are targeting come back in the content of your site, this will save you a lot of time on link building. If it doesn't, just try to add them to your content one or two times is enough depending on your backlinks, you do not need to repeat the keywords over and over.