A system or a process "comes online" when it begins its normal operations. In today's lesson, you heard that Europe is constructing new facilities to regasify imported liquified natural gas. However, those facilities won't come online for a few years. That means, those facilities won't begin operations for a few years.
This can be used for computer systems too. For example, you could say that a company is investing in new computer system to manage its inventory and sales. The system is scheduled to come online in about six months.