An oil slick is a way to put a barrier between the dough and the table to prevent sticking. Many bakers use flour to stop their dough sticking when hand kneading, but this causes the flour to get absorbed into the dough.
This is bad news as the bakers recipe alters without control as it now the dough includes extra flour. It also causes flour to enter the dough that has not undergone fermentation so is raw.
Raw flour can make the bread taste dusty, make holes throughout the crumb and ruin the rise. We want to avoid it as much as possible.
Especially when baking in hot conditions. The dough can get stuck everywhere! Bakers can be tempted to dust flour on the tables even more so than normal which causes the same issues.
To avoid the issue of incorporating extra flour into the dough we can use an oil slick. By this we take a small amount of olive oil and rub it on the work surface. Some tables such as a bare wooden table will absorb more oil than coated tables.
This technique can be used in the beginner's bread recipe, here's a video on how it works: