RPM packages can ``depend'' on other packages, which means that they require other packages to be installed in order to run properly. If you try to install a package for which there is such an unresolved dependency, you'll see:
$ rpm -ivh bar-1.0-1.i386.rpm failed dependencies: foo is needed by bar-1.0-1
To handle this error you should install the requested packages. If you want to force the installation anyway (a bad idea since the package probably will not run correctly), use -nodeps on the command line.