The function will only return the current index of the active loop. Basically you cannot access fields in a repeater without having an active have_rows() loop. This works the same as the WP have_posts() loop. Without a loop, many of the functions in WP will not work because they are only designed to work inside the loop.
// accessing only the first row of a repeater
if (have_rows('repeater')) {
while (have_rows('repeater')) {
the_row();
the_sub_field('sub_field_name');
// after doing what you want to do with the fist element
break;
}
}
// if you want you can reset the above row to
// begin from the first row again
reset_rows();
// start another loop on the same field
// if you don't use reset above then it will
// start with the second row
if (have_rows('repeater')) {
// etc....
}
