What exactly is a Life Size converter?

Can someone explain what a life size converter is and when/how it is used?

I guess you are refering to the life size converter that is meant to be used with the canon EF50mm macro lens.

This particular item is an odd one.It looks like an extension tube but has optics like a teleconverter.The whole point of it is that it doubles the macro magnification to 'life size'
So what is life size? It's the way macro lenses are measured for magnification -

Because photo's are enlarged by different amounts ,a way was needed to describe the magnification performance of macro lenses.The most sensible was to compare the size of the object being taken to the actual size it appears on the film/sensor.
So life size means that a bug that is 10mm long will appear 10mm long on the film or sensor at 1:1 macro ratio.

One other thing you should know is that the canon 50mm macro lens is one of the very few macro lenses that needs a life size converter to achieve 1:1 macro. Most true macro lenses can do 1:1
If you were considering buying the canon 50mm macro ,have a look instead at the sigma 50mm macro lens. The sigma does 1:1 without any silly add-ons and has a GREAT reputation for image quality

Thank you Ron. I understand and that makes perfect sense.

Note that although the life size converter was designed for use with the 50mm macro in mind, it can be used with any lens but you will lose infinity focusing.