NB - this applied to Mambo before the Joomla developers forked off; I've read that Mambo has since included SEF URLs, tho maybe an imperfect method (I haven't tried).
Mambo pages are compiled on the fly - as they're requested by browsers, and their original URLs (addresses) reflect this: they're stuffed with code, numbers, characters like "&". Looking like gibberish to most humans, they can also be rather tough for search engines to follow as they try navigating through sites.
There are, however, improved URLs: the search engine friendly URLs, or SEF URLs. But these are by no means created equal; I've found that some "SEF" URLs are friendlier than others. And with latest Mambo version - and with Joomla!, can get lots of Search Engine Unfriendly URLs (due to ItemIDs). Can be bit of nightmare, without care, trying to get truly SEF URLs - with one URL per item.