What is "tws_delay" and how can I use it? (with an ATTiny85)
tws_delay() is a function provided by the TinyWireS library here. As you can see from the declaration - void tws_delay(unsigned long); - it accepts a parameter of unsigned long, just like delay() does.
In fact, the comment on the line above that declaration says...
// Implement a delay loop that checks for the stop bit (basically direct copy of the stock arduino implementation from wiring.c)
... so, it's safe to say that wherever you would have used delay(), you can use tws_delay().
So there should be no need to declare or define it. But to simply use it. If using tws_delay() in code that includes TinyWireS.h doesn't work, I would be checking that you have a version of the library that declares/defines it. Which you should, given the example you linked, as that repo has had the function since 2012!