To inoculate a wine is to add an active yeast culture to the must (juice) to activate fermentation.
Winemakers choose certain known strains of yeasts to achieve desired results. You may have heard the term ‘native yeasts’ used in the wine world at some point on your journey. What is being referenced is a native strain of yeasts to the area the winemaker is growing their grapes.
At some point in the winemaking process, a winemaker may want or choose to soften their wine through malolactic fermentation. This is the process by which they inoculate with a malolactic bacteria starter. Malolactic fermentation takes the harsher, more biting malic acid and converts it into lactic acid which is less abrasive and softer on the palate. Most all red wines go through malolactic fermentation, but the best way to identify it is trying a Chardonnay that you know does not undergo malolactic fermentation alongside the traditional California styling that does see malolactic fermentation. They are two completely different wines, from the same grape.
Natural fermentation and natural malolactic fermentation can occur without any inoculation, and some winemakers desire the results of that natural method for their winemaking process.