Problem: you've moved your website. For various reasons the urls of existing pages may have changed.
Solution: a .htaccess file at the root of your old site which redirects old urls to new ones.
But there are a large number of such pages. Matching them up and compiling the redirect file is going to take time....
404bypass scans your old and new sites, 'smart matches' the pages and offers you a table of results. You can make any corrections manually before generating a .htaccess file, or a csv (or any other type of file you like, using a template configuration system). There are more details and screenshots here.
The beta period has come to an end, 404bypass is on full release now. For a limited period, it's available at an introductory price of $5 ($10 when it reverts to full price).
Download and try it here (30 day trial).