I would suggest reading the "Background" tab for your experiment carefully. It provides a pretty good explanation of the role of measuring pH in this experiment. I know it may seem like a lot and may be overwhelming, but it is very important to read the entire lab first before you start. You never know, maybe you will miss a crucial step or make a mistake on the timing because you did not read it first.
From a quick glance at the descriptions, it seems to me like you would be dissolving solid lead to form lead ions by using vinegar. The vinegar solutions you would use vary in their acidity. This is where the pH comes in--to measure acidity.
The goal of this chemistry science fair project is to determine how varying the pH of the test solution affects its ability to dissolve lead, which is a critical step if the lead is to be successfully detected.
This seems like a very fun chemistry project! Good luck on it and please don't hesitate to ask if you have anymore questions.