Just-in-time compilation of Ruby allowing you to elide a lot of the overhead of dynamic language features + executing optimized machine code instead of running in the VM / bytecode interpreter.

For example, doing some loop unrolling for a piece of code with a known & small-enough fixed-size iteration. As another example, doing away with some dynamic dispatch / method lookup for a call site, or inlining methods - especially handy given Ruby's first class support for dynamic code generation, execution, redefinition (monkey patching).

From https://railsatscale.com/2023-12-04-ruby-3-3-s-yjit-faster-w...,

> In particular, YJIT is now able to better handle calls with splats as well as optional parameters, it’s able to compile exception handlers, and it can handle megamorphic call sites and instance variable accesses without falling back to the interpreter.

> We’ve also implemented specialized inlined primitives for certain core method calls such as Integer#!=, String#!=, Kernel#block_given?, Kernel#is_a?, Kernel#instance_of?, Module#===, and more. It also inlines trivial Ruby methods that only return a constant value such as #blank? and specialized #present? from Rails. These can now be used without needing to perform expensive method calls in most cases.

it makes ruby code faster than c ruby code so they are moving toward rewriting a lot of the core ruby stuff in ruby to take advantage of it. run time performance enhancing makes the language much faster.

Same as the benefits of JIT compilers for any dynamic language; makes a lot of things faster without changing your code, by turning hot paths into natively compiled code.