Stabilizing is treating a finished cider or mead — usually with potassium sorbate and/or potassium metabisulfite, or alternatively through pasteurization — to prevent any residual or subsequently-added yeast from fermenting further. It's the specific step that makes back-sweetening safe rather than a genuine over-carbonation risk.
Why sorbate alone isn't always enough
Potassium sorbate prevents yeast from reproducing but doesn't necessarily kill an already-established, actively fermenting population outright — it's most reliable when fermentation has already slowed or stopped naturally before it's added, which is why sorbate is commonly paired with potassium metabisulfite (which suppresses yeast activity more directly) rather than relied on alone, especially in a must that's still showing any real fermentation activity.
Confirming fermentation is actually finished, not assumed finished
Stabilizing before fermentation has genuinely completed — rather than just appearing to have slowed down — risks a batch that still has active yeast working through it, undermining the whole point of the stabilizing step. Taking stable gravity readings across several consecutive days, the same check used before bottling a beer, is the right way to confirm a cider or mead is actually done before stabilizing and, afterward, back-sweetening it. Skipping that confirmation step is the single most common cause of a stabilizing attempt failing to actually stabilize anything.