The viscosity of an oil is its ability to resist flow and is measured in mm²/s. To classify this, oils are classified with two indices.
The index before the W (Winter) is the cold viscosity index allowing the engine to start cold. There are 6 cold viscosity indices (0W-5W-10W-15W-20W-25W).
The index after the W is the hot viscosity index (measured at 100°C). It characterises the thickness of the oil film when hot. There are 4 hot viscosity indices (30-40-50-60).
In both cases, the lower the viscosity index, the more fluid the oil will be.