Answer:
Its a technique used to reduce the number of colonies that could grow in a culture. 1ml of the sample specimen is diluted into 99ml of saline then 1ml of that solution is put into a 2nd tube that has 99ml of saline and finally 1ml of the 2nd solution in transferred into a 3rd test tube with 99ml of saline.
The first dilution (the one with the original sample) is diluted 1/100 the second 1/10,000 and the 3rd one by 1/1,000,000