Ian if you get a mo

Explain ISO for me in English

And in less than 150 words lol
I think i understand it? But not 100%
I know if its bright and a want a fast action picture frozen in
time (eg wheel spinning) i up the ISO
But as it gets dark or dull light then ISO comes down to 200 or 100
that's why i said "Did you say 2000" because my jaw dropped lol...
Is it the camera or the man behind it
