Blogs change over time -- posts and pages are added and deleted. However, it can be hard to keep track of all the links that point to URLs that no longer exist, especially if there are a lot of links to your blog from other blogs and websites. The Custom Redirects feature allows you to take an old URL, and point it to a new one.
Let’s say you have a recipe blog, and one of your most popular posts is about banana pancakes. A year or so later, you decide you only want your blog to be about gluten-free recipes, so you go through and delete all the posts with recipes involving gluten (including the popular recipe about banana pancakes). However, since your recipe was popular, other blogs or sites had links pointing to your post. After you remove the recipe, anyone trying to access the page receives a “Page Not Found - 404” error.
With Custom Redirects, you can enter the old URL that used to lead to the banana pancake post, and redirect it to another URL (perhaps the address of a recent post you wrote using gluten-free flour and milk).
To enter a Custom Redirect, go to Settings | Search Preferences and click Edit next to "Custom Redirects." Enter the old URL in the "Source" field, and the new URL in the "Destination" field. Click Save changes, and you’re done! To add more redirects, select New redirect.