I just did a search of the VPython documentation and found (and fixed) two obscure places where the keyword "wait" was mentioned. If you see one I've missed, please let me know.
I'll explain that in JavaScript, the browser programming language to which a VPython program is compiled (using the RapydScript-to-JavaScript compiler plus some preprocessing), if you write an infinite loop that is a common structure in VPython programs, the browser locks up -- no updates to the web page are possible while the loop is running. GlowScript uses the Streamline library to restructure such "synchronous" programs to make them "asynchronous", by moving the loop contents to a separate function that is then called repeatedly. A loop containing rate(200) is transformed into a function that is driven by a timer 200 times per second.
Streamline needs to be told that code needs to be restructured before being executed, and the hint is the keyword "wait". As a convenience to those writing GlowScript VPython programs, the preprocessing that is part of compiling a VPython program looks for statements such as rate and sleep and scene.waitfor and inserts the keyword "wait" if it is not present.
Note that this means that one should not name a variable "wait", which will confuse matters.