Just in case you can't find it, or find the wikipedia/google articles confusing (as I did at first). I'll give you the basic rundown.
RPM is two things, the Redhat Package Manager (used by RedHat Linux, Fedora, openSUSE, CentOS, and several other distros), and .rpm, the file-format used by the package manager.
A package manager is a program which makes it very easy to install, update and uninstall programs. The packages are kept online in what's called a repository (repo or rep for short). The package manager gets a list of all the programs, which you can search. You can search and select programs from the list to install. It will then automatically download and install the programs you've selected, and any programs that it needs to run (called dependencies).
The system of repositories and package management works very well with open-source, because the repositories have 10-20000 pagkages in them, so if there's a program you want, chances are it's in the repository.
Note that there's some finicky difference between rpm, which is a package manager, and the various dependency managers, ie yum, apt-rpm, yast, etc. The package manager deals with taking the compressed files and installing them into the right folders on your computer, and the other deals with downloading and installing all the packages that a certain package needs to run. At this point in your linux adventures, you don't really need to worry about the difference.
Hope that helps!