AltJIT allows apps sideloaded with AltStore to enable just-in-time compilation, more commonly known as JIT. JIT allows some apps to run significantly faster β or even at all β but iOS does not normally allow apps to use JIT for security reasons. AltJIT is an AltStore-specific workaround that allows you to enable JIT for any sideloaded app whenever you're on the same WiFi as AltServer.