The dictionary definition of reify is “to consider or make some abstraction more concrete or real”.
The usage in the features section is meant to convey that everything in a Clojure program can be manipulated in the REPL with Clojure itself: we can look at the source, at the metadata, at the types behind any object, at all aspects of the data itself – and we can modify definitions on the fly, changing or building the program up dynamically, live in the REPL.
Many (most?) other languages don’t provide all of this dynamically in a way that you can inspect and modify it all, in a running program, using the same language and semantics that you are writing your program in. If they provide it at all, you often need to use additional tools or additional semantics that are outside of the core programming language.
Stu Halloway has a great slide in his REPL-Driven Development talk, about six minutes in, where he compares the Clojure REPL to the interactive Java shell as an example of the dynamic aspects of Clojure compared to the “pour concrete” of many other languages.
See talk-transcripts/REPLDrivenDevelopment.md at master · matthiasn/talk-transcripts (github.com)