example:
for each integer a from 1 to 10 do ......
so integer a represents those integers 1 through 10. you can use it however you want, for example in variable arrays:
for each integer a from 1 to 10 do <set some_variable[integer a] = unit-type of (last created unit)>
so here, some_variable[1] through to some_variable[10] will contain the value of the last created unit-type
same story goes for integer b.