When you create pages in mojoPortal, the page gets a friendly url based on its title, like /fun-stuff.aspx would be the url for a page named Fun Stuff. Now if you change the name of the page later it will get a different url, so for example if I change the name to Really Fun Stuff, its going to get a new url /really-fun-stuff.aspx.
Now from an SEO (Search Engine Optimisation) point of view, its not a good idea to be changing your urls willy nilly. If someone bookmarked your old url you don't want it to be broken, or if its already popular in search results, you don't want it to be a broken link. So ideally, you need to have the old page do a 301 redirect to the new page. The 301 status code tells the browser that the page moved permanently. Now mojoPortal will create the 301 redirect for you automatically when you re-name a page or blog post. However, its still not a great idea to be renaming your pages frequently, you should avoid it generally but if you need to its ok to do it. You can also see the 301 redirect mappings and manually manage them from Administration Menu > Advanced Tools > 301 Redirect Manager. You need to be very careful with this feature, you do not want to create circular redirects where 2 urls redirect to each other.