This announcement post is worth reading if you're interested in the technical details, but for 99% of developers shipping to the App Store ABI stability will have no impact on the way you work. Your users will save some disk space but that's about it. All of that said, I'm not trying to downplay this milestone. It's a great step forward for the maturity of the language!
Oh and if you distribute a framework/library in binary form, this is huge news and you'll be able to finally start using Swift! 🎉
UPDATE Oops! 🙈 I made a mistake with my last comment about distributing libraries, read in the post about Module Stability for more information on this. Thanks to Fabian for pointing this out.
One change in Swift 5 that may have more impact on your code is this one. I'm not going to try and summarise this too much here because it's a complex subject and I wouldn't do it justice. But suffice to say that runtime exclusive memory access may require some code changes in your project, and may also slow your app's performance down a little.