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A Lamb is a derogatory term used by Solarians to refer to non-Solarians (land/surface/earth dwellers) who live on the planet. At one point, Shevat queen Zephyr uses the term "land dwellers", so it is unknown if "Lamb" also applies to Shevatans.
Approximately 200 years after the Shevat-Solaris War ended, in 9698, Karellen forcibly put limiters into all the Lambs to prevent any future uprising. How this was accomplished is unknown, but it could have been done by the Limiters in the meat, food and medicine in the Soylent System being fed to the populace.
The opposite of "Lamb" is the "Abel", which Solarians call themselves. They also call themselves "Shepherds".
The Lambs are being monitored by Solaris and the Gazel Ministry, and it is implied these are what the mysterious yellow save points are for. Lambs are secretly given an ID known as a "Lambs Number".
Lambs are baby sheep. It may be a metaphor for "sheeple", in reference to how brainwashed the majority of humanity in Xenogears is.