Where can Cydia be found?
They have dominated the scene for a few years now and have proven to be beneficial with updates and jailbreaks. Most of the jailbreak for iOS 10 will be listed on iOS 10 Jailbreak page.
What exactly is Jailbreaking?
In a nutshell, jailbreaking is simply going past some security layers on Apple devices using a simple software. It takes advantage of the vulnerability that iOS exhibits are removing all restrictions set by Apple on its platform. This allows you to use Cydia on your device.
Apple is always on the watch to try and seal holes that have been exploited by jailbreaks by constantly updating its platform, but this in return creates another hole that can be exploited. This is the reason why there is no single jailbreak for all iOS versions.
How safe is Cydia?
In the beginning when Cydia was launched its safety was a worrying issue due to its instability, but as time has passed by, improvement has been made. Still, like any other software that depends on jailbreak, you always have to be taking chances. Unlike when you download an app from Apple app store where apple tests all the apps before allowing them to be in the app store, you do not get the assurance when you download from Cydia. Once in a while though not often, a rogue tweak finds its way into Cydia. So the only advice is that you be careful. Use default repositories as often as you can since they are guaranteed to be safe.
The other problem you may face is compatibility since there are a lot of tweaks on Cydia chances, are some may not be able to work alongside each other and can damage your device. The benefits of jailbreak and Cydia may out a way the risks; try on and enjoy.