Leaving mainstream support only means that Windows 7 will not be receiving any new features or product tweaks, such as DirectX 12 gaming technology slated to launch with Windows 10. Free software support from Microsoft is not going to happen either.
Once a Windows desktop operating system leaves mainstream support, it enters the extended support phase. Windows XP was in that state from early 2009.
Users will still get security patches during extended support, which means that Windows 7 will not be cast out completely yet. Hotfixes will still be provided, too, assuming they are security related.
Extended support for Windows 7 lasts until January 14, 2020 so if you are happy with all that, there is no reason to rush to upgrade.
However, it is a sign that Microsoft has given up on the operating system, which is subjectively seen as being better than its successor.