Can anyone tell me what is exactly a bore?? and what’s the difference between them…
Well, it’s some people you know and often wish you didn’t, and the difference between them tends to be one of degree of boringness - all the time/some subjects/even when asleep etc 
Alternatively, it’s the width of the orifice inside your whistle tube…wide/standard/narrow which affects the volume and responsiveness of the whistle amongst other things. I don’t think the system is standardised, but maybe one of the technical team will chime in with a range of parameters 
Trisha
While the number changes depending upon who you talk to, an ideal bore to length ratio would be around 1:23. With a lesser ratio generally favoring the lower octave and a higher ratio generally favoring the upper.
To say it another way, the size of the bore generally effects the volume and note stability. Narrow bore typically handles the second octave better and are a bit quieter. Wide being the opposite.
Erik