Chained fixups is a new way to store information that will be used by dyld. Replacing LC_DYLD_INFO(_ONLY), the chained fixups can save binary size and reduce launch time.
Traditionally, dyld, at launch time, needs to slide the fixed addresses with a random number, known as ASLR. This operation is called rebasing. Also, dyld needs to connect symbols from current binary with its linked dynamic libraries. This is called binding. Under the new format, both rebasing and binding have a new name, fixup, because they need to be "fixed up" before main function.
Chained fixups is enabled by default if the binary is built for device and targeted at iOS 14+. We can also manually enable it by passing -fixup_chains to ld. To disable it, use -no_fixup_chains.