It's hard to know what else it could possibly mean, but the Washington Post dutifully reported to readers:
"The aging population is shrinking here, with the 2011 census showing a loss of about 1.5 million people since the 1980s. As the decline accelerates, by 2030 the government predicts a hole as big as 2.3 million workers in the German labor force."
In a market economy, wages adjust to equilibrate supply and demand. If there are fewer workers in Germany it means that workers will leave less productive sectors, like retail trade, restaurants, and hotels, and instead move to sectors in which they can get a higher wage. Many of the firms in the less productive sectors will go out of business.
This sort of transition happens all the time. It is the reason that half of the U.S. population is not still working in agriculture. In the context of a market economy it is not clear what it can mean to have a hole in the labor force. This would just mean that low productivity jobs cease to exist. So what?