Full ratchet anti-dilution recalculates a prior round's effective price as if the round had happened at the new (lower) price per share. A Series A investor who paid $10/share would, after a full-ratchet adjustment to a $5/share down round, have their conversion price reset to $5, effectively doubling their share count.
Full ratchet is the most punitive form of anti-dilution and is rare in modern term sheets. Weighted-average anti-dilution is the standard. When full ratchet does appear, it typically signals a hard-fought negotiation in a difficult market.