although I am sure you could find this with some searching.
i is a variable set up to inside the loop (in this case an an int type) to keep track of how many times the for loop has been repeated thus far.
int i=0 //sets an initial value
i<20 // if this statement is true run the loop again
i++ // add one to i
and yes. it will run 20 times.
first time i will be 0
second time i will be 1
........
........
........
twentieth time i will be 19
then on the twenty-first time i will be 20 which makes i < 20 false and the loop exits.
the end.