When it comes to motor oil, the thickness of the oil is indicated by its weight. The lower the number, the thinner the oil. So in terms of thickness, 5W oil is thinner than 15W oil.
The main purpose of motor oil is to lubricate and cool the engine. It does this by forming a thin film between moving parts and by absorbing heat. The thicker an oil is, the more resistant it is to flow. This means that thick oils can provide better protection at high temperatures, but they can also cause increased friction and wear at low temperatures.
For this reason, most passenger car engines use a light-weight motor oil such as 5W-30 or 10W-30 in winter and a slightly heavier weight such as 15W-40 in summer. Using a too heavy oil in winter can increase fuel consumption and cause damage to cold start components, while using a too light oil in summer can result in increased engine wear.