This can mean garden compost made from waste materials rotted down in a compost heap, but it usually refers to the special soil or peat mixtures used for sowing and potting plants.
There are two main kinds. Soil-less compost is made from peat or a substitute such as bark or coir. Soil-based composts are a mixture of sterilised soil, peat or an alternative, and sand. They all have added fertilisers.
A universal, soil-less compost is suitable for all normal sowing and potting needs, but there are different grades of soil-based compost. You can also buy special composts for rooting cuttings or for growing ericaceous (lime-hating) plants, orchids, and water_plants.
Make sure the compost you use is moist, warmed to room temperature, and fresh. Don't keep left-over compost from one season to the next as the fertilisers sometimes change into harmful chemicals with age. Instead, use up your old supply to improve the soil outdoors.