What is a 302 Redirect?
A 302 redirect is a method of redirecting from one URL to another. It gets its name because the HTTP Status Code returned by the URL is 302. Originally, this status code meant "Moved Temporarily", but now means "Found".
Search engines and browsers generally interpret this message that the redirect is temporary phenomena, and could change back at any time. Search engines will not remove the original URL from their index, and will not pass any ranking value (or "link juice") to the target URL since they expect this situation to be temporary.
Why It's Important
The problem with 302 redirects is that they're often used incorrectly, and can be harmful for the SEO of your site. If the URL redirection really is a temporary situation, then you're fine, and nothing needs to be done. However, the majority of redirects are not temporary, thus 302 redirects should not be used.
Since no link juice is passed to the target URL, if you have a high-ranking URL that you move to a new domain or a new location somewhere else on your site, all of the hard work you've put into getting the first URL ranking well will not be passed on to the new URL location. This can be disastrous for your site's rankings.
How to Fix
Instead of using a 302 temporary redirect, a 301 "Permanent Redirect" should be used. 301 permanent redirects typically pass all the original ranking value from the first URL to the target URL1. Therefore, when a page moves, it's possible to keep the same ranking position - thus preserving your SEO efforts. It's also a signal to search engines that the original URL is no longer used, so they will generally remove it from their index.
Implementing a 301 redirect is different for each web server. If you're running an Apache web server, it can be done in the .htaccess file. For Windows servers, the redirects can be set up by creating a redirect rule in the Internet Services Manager.
1. Most search engines will pass all or most of the original ranking value to the new URL. However, if there are too many redirects in a chain, it's possible that search engines may give up following the redirects and all of the value will be lost, and the target URL will not be indexed. While some search engines such as Google say that they will pass all of the ranking value from a 301 redirect, it is not precisely known if other search engines will deprecate some of this ranking value passed via 301.