In the tech world, upgrades are usually a good thing. IOS fixes bug fixes and security patches that protect and keep your iPhone up-to-date. However, in the jailbreak world, upgrades can mean certain death, as we recall today, that Apple did not sign iOS 11.4 anymore. That means if you upgrade to 11.4.1, you will not be able to jailbreak your iPhone anymore.
When Apple stops signing software, iTunes does not recognize it or install it on your iPhone. This is usually not a problem, as you do not usually have to go back to an earlier version, but jailbreakers need to stay a step or two behind Apple, as it takes some time to exploit iOS. Right now, jailbreak developers working on iOS 1
There are ways to solve this problem, but it takes some foresight and planning ahead. You can save your SHSH2 blobs to reset your iPhone to iOS 11.4, even if Apple no longer signs it. This method "tricks" iTunes into thinking 11.4 is still signed, so you can downgrade to a version of iOS that is compatible with the most stable jailbreak available today. If you're still using iOS 11.4, think about saving your SHSH2 blobs as a jailbreak insurance.
If you've already upgraded to 11.4.1 without saving your blobs, all your hopes will not be lost. While you can not jailbreak your iPhone as it is, developers might find an exploit to jailbreak iOS 11.4.1. However, it is possible that this version will be completely skipped for iOS 12, as Apple's next major software release – currently in beta testing – is just around the corner. However, in preparation, you can now save these blobs before Apple stops signing 11.4.1.