Cutting oil is used to lubricate glass cutters, it helps breaking and longevity of your cutting blade. Use either in an oil filled glass cutter (e.g. Toyo or K-Star or our new CGG value glass cutters) or by dipping your cutting wheel in oil prior to scoring.
Cutting oil also works by filling the score that your cutter has made, which since glass is a semi-liquid material to a small extent stops the score closing over. This is a small but important benefit in using oil when cutting glass!