Upconverters give the rtl-sdr the ability to receive frequencies below it’s minimum frequency. This allows reception of things like AM broadcasts, ham radio signals, weatherfax, international radio and much more.
There are dozens of upconverter designs and commercial products that work with the rtl-sdr. Luckily to help you choose one, blogger and amateur radio hobbyist KF7LZE has posted a rather extensive review of all the upconverter options available for the rtl-sdr.
The article can be found here.