Basically, providing that you haven't disabled Ultrapeer capabilities in the Options, then you become an Ultrapeer when the connection says Turbo-charged. A Turbocharged connection means that you are connected to 6 other Ultrapeers. If you are connected to less than that then you are a leaf node.
As to what an Ultrapeer does, it's just as the description says: an Ultrapeer will "shield" a leaf from the majority of messaging traffic, or, in other words, will filter what messages are sent your way (in the case you are a leaf). That is because your connection to the network (Gnutella) is not as good as an Ultrapeer's and this way, you won't get overwhelmed by those obsolete (for you) messages. Sharing many files can help get a better connection because it will make sure that more people will try connect to you to dld your files, so you'll get more connections to Ultrapeers, hence a better connection status. It is, of course, not the only factor to consider.
Does that make it any clearer?

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