It's the powerpc emulator needed to run PPC programs. It should ask you if you want to install it (assuming you are on Snow Leopard, where it isn't installed by default). It's not compatible with Lion (i.e. Apple have dropped support).
http://docs.info.apple.com/article.html?path=Mac/10.6/en/27987.html
You can also install Rosetta off your 10.6 install DVD. Here's the help article for reference.
What leopard installation disk or applications??
Rosetta is a PPC (PowerPC) program emulator for OS X. It is available on your OS X install DVD. I think there also may be a web download provided by Apple when you attempt to run a PPC program.