The indicator dots are located between the foul line and the arrow zone. They are placed on the lane to help bowlers play angles.
Angles are when the ball is released over one board at the foul line and a different board at the arrow zone. For example if a bowler releases the ball over board 12 at the foul line and rolls it over board 10 at the arrow zone it is an angle because the two board numbers are different. Since the indicator dots are half way between the foul line and arrow zone, the ball would roll over board 11 in the previous example (11 is between 12 and 10).
Since the bowler knows where the ball will cross at the indicator dots and the arrow, the bowler can visualize a path the ball will travel instead of one distance down the lane. Each bowler can adjust the distance they initially look (closer or farther) to accommodate what works best for them.