SecondBar is a tool which allows you to have more than one menubar in Mac OS X. This is useful if you have multiple monitors connected to your mac.
So if you are working with a program at a screen without menubar and want to use the menu you have to move your mouse all the way back to the menubar (just to move it back to the other screen a few moments later).
This thing really helps you out when you have to work on Mac with multiple montitors: Apple seems to be doig its best not to allow the users use more than single display because it would ruin their Lion tablet metaphor.