Preparation of soil by mechanical agitation.
Tilling covers a range of different techniques to prepare soil for planting, by using mechanical agitation. Human / horticultural techniques include shoveling, picking, mattock work, hoeing, and raking.
Primary tilling will tend to turn a large amount of soil over. For example a gardener might "dig deep" to dig compost into the soil, or a farmer might plough old stubble in, to increase the humus content.
Secondary tilling (eg. with a hoe) produces a smoother, loose soil bed, ideal for sowing.
use this tag for all questions about tilling: which method should be used, alternatives, etc.