Soil sieves (or riddles) are useful for sifting out large lumps from the soil, leaf mould, and compost, to leave you with a fine material suitable for sowing seeds like salad leaves and sunflowers, or for potting mixes.
It is a device for separating wanted elements from unwanted material or for characterizing the particle size distribution of a sample, typically using a woven screen such as a mesh or net or metal.