YForth? is an interestingly small implementation of Forth for Linux. It suffers several deficiencies. For general Forth programming or learning under Linux, yForth? is not the best choice.
From the author's README:
yForth? is a Forth environment written entirely in ANSI C, making it extremely portable. The first thing I want to tell you about yForth? is that it seems a joke compared to other systems such as gForth or PFE.
Nevertheless, you could find yForth? nice, in which case you're invited to explore yForth?
It's yours, you can make anything you want with it. If you want an explanation of the words provided by yForth? please refer to the draft of ANS Forth or something equivalent. The Net will help you.
Do not expect the prompt "ok" to come up when you run yForth?, the standard says that "ok" shall be printed AFTER every successful command execution...