Hi Shubham,
The lookahead distance determines how far ahead the target location would be for the UAV along the path defined by waypoints. Check the illustration below, the larger L is, the further away target point would be, which usually yield a more consistent flight direction for the UAV. If L is too small, the UAV would need to adjust its heading frequently. If L is too high, the UAV might not converge to the path line fast enough.
Thanks,
Jianxin
