Facebook lite is the version of Facebook app which consumes less data as well as less space on the phone.
The Facebook lite app was developed by Facebook for mobiles which has less ram or the user wants to save data from his cellular data pack.
The Facebook original app consumes a lot data as well as space on the phone so the Facebook lite app gives same experiences as Facebook app but consuming less data as well as space on phone.
So if you have phone with less ram and storage or you want to save your data go ahead and try facebook lite app.