Slow steaming refers to the practice whereby the (operational) speed of the ship is reduced. It basically means that the ship’s engine is not used at full power, thus saving fuel, reducing CO2 and air pollutant emissions.

Reducing ship speed by 10% will lead to a 27% reduction of the ship’s emissions. Overall, if all ships were to slow-steam, the available capacity on the market would be reduced (more ships would be needed to carry out the same transport work). If the additional emissions of building and operating these new ships were considered in the equation, then reducing the fleet’s speed by 10% would lead to overall CO2 savings of 19%.

Reducing the (operational) speed of ships multiplies the positive effects of an energy efficiency index, as it results in burning less fuel and therefore emitting less CO2 and other greenhouse gases. It also contributes to significantly lower emissions of air pollutants such as NOx and PM, with benefits greatly outweighing costs. Slow-steaming is often regarded as the most cost-effective way to reduce CO2 emissions as it can be done at almost no cost while translating into operational savings.

Source: https://www.transportenvironment.org/what-we-do/shipping-and-environment/shipping-and-climate-change