The phrase "reliable way" means a method or approach that you can trust and depend on to consistently achieve a certain result or outcome. Full definition
Online dating is one of the most reliable ways of meeting other singles either within your area or in other parts of the world. (internationalxdating.com)
The scientific method is the most reliable way of understanding the natural world. (scienceornot.net)
So if you need a certain amount of money at a specific time in the future, the CD is a more reliable way of getting it. (money.stackexchange.com)