It introduces a number of new features and performance improvements, most notably:
A new JIT compiler.
The RubyVM::AbstractSyntaxTree module.

The JIT compiler aims to improve the performance of Ruby programs. Unlike traditional JIT compilers which operate in-process, Ruby’s JIT compiler writes out C code to disk and spawns a common C compiler to generate native code. For more details about it, see the MJIT organization by Vladimir Makarov.

Ruby 2.6 introduces the RubyVM::AbstractSyntaxTree module. Future compatibility of this module is not guaranteed. This module has a parse method, which parses the given string as Ruby code and returns the AST (Abstract Syntax Tree) nodes of the code. The parse_file method opens and parses the given file as Ruby code and returns AST nodes.