What is secondary fermentation?

After the primary fermentation, most wines go through a second fermentation.  This is called malolactic fermentation, although it is typically referred to as "ML".

Virtually all red wines, and many white wines are put through this additional fermentation.  It is the conversion of malic acid (think tart green apples) to lactic acid (think milk).  This is accomplished with a different strain of bacteria than primary fermentation, and it takes a little longer.  (Sometimes it can take all winter!)  As opposed to primary, the "ML" fermentation is an anaerobic process.

All red wines will ultimately go through ML either by being coaxed or on their own.  If the mistake is made of bottling before ML is complete, it is likely the corks would pop out of the bottles, or worse, the bottles could explode.

If you have ever had a red wine that was slightly effervescent, it is likely that the wine was bottled prematurely, but not early enough in the process to cause disastrous results! 

In white wines (or reds that do not go through ML) the addition of sulfites normally will block the process of ML starting up in the bottle.  However, most wineries will put the wine through a sterile filter to remove any bacteria which could cause unwanted fermentation.

ML gives wine a much smoother taste and mouthfeel, and is a key technique in wine making.