while searching seo terms i found this bad neighbourhood and curiouse about it !
what actually we meant for vad neighbourhood and what are in case of site ?

Bad neighborhoods are sites that aren't real content - they're just intended to increase page rank. If a site is identified as a bad neighborhood, and they link to your sites, you can be penalized. This is to discourage schemes to manipulate the search engines.

Black Hat SEO - Web Spamming and Linking to Bad Neighborhoods

I didn't realize that you should keep a constant eye on the sites you've linked to in case one of them becomes defined as a bad neighbourhood.

Thanks for the link to that article.

You're welcome. It's a good idea to avoid free link exchange websites for this reason. Make sure you check out any site that you link to to make sure it's not just a bunch of spam and links.