Lead Free Solder
Lead Free Solder is typically an alloy containing Tin and other metals, though some options do exist that contain no Tin but these are not used within glasswork. Lead free isn't often recommended to be used unless absolutely necessary as it's more difficult to work with, more expensive and the primary point of lead isn't terribly important in something that will rarely see any interaction with people. Though if something will be interacted with, then it's advised to use lead free solder.
Alloy Labeling
Due to lead free alloys not having a fixed set of metals, many alloys simply do not have a standard. They are referred to by a trade name or just "Lead Free" depending on how many options that manufacture has.