In a 2000 interview (that is, pre-YARV), Matz said
Matz: I'd like to make it faster and more stable. I'm planning a full rewrite of the interpreter for Ruby 2.0, code-named "Rite". It will be smaller, easier to embed, thread-safe, and faster. It will use a bytecode engine. It will probably take me years to implement, since I'm pretty busy just maintaining the current version.
What was meant by "thread-safe" in this context? An interpreter that allowed you to use green threads? An interpreter that allowed you to use native threads? An interpreter that didn't have a global interpreter lock (GVL in YARV Ruby terminology)?
At the moment ruby's threading is less than ideal. Ruby can use threading and the threading works fine, but because of its current threading mechanism, the long-and-short of it is that one interpreter can only use one CPU core at a time; there are also other potential issues.
If you want all the gory details, This article covers it pretty well.