When dealing with cover crops, green manures, and in other things I see the term 'Allelopathy', especially regarding replanting an area. What does this term mean?
The word 'allelopathy' is defined as 'the chemical inhibition of one plant (or organism) by another, due to the release into the environment of substances acting as germination or growth inhibitors'. The term first appeared in the 1930's, coined from the Greek derived compounds allelo and pathy, meaning respectively one another or each other and mutual harm or suffering - therefore, the term allelopathy, in horticulture certainly, always refers to deleterious effects on plants. Separate from that, from Man or a grower's point of view, there may be a perceived benefit if certain weeds or unwanted plants are suppressed because they're being affected by allelopathy.
Bracken is a plant associated with allelopathic behaviour, and there is some evidence to support it; there is also a suggestion that many ferns, and certainly many grasses, all use allelopathy to reduce competition from other plants, but the amount of research into those is pretty small. Research is ongoing, though primarily with regard to how mankind can benefit from allelopathy, more reading here
Basically allelopathy is a mechanism where one species of plant affects the growth of another via chemicals (typically exuded from roots). It can be both positive and negative, although gardeners usually use the term in its negative (growth inhibiting) context. A classic example is the black walnut tree which produces a chemical, juglone, which inhibits many species from growing beneath the tree.
Inhibition might be general or quite specific - I would very much like to get hold of some of a variety of marigold which is claimed to prevent convolvulus growing in its vicinity. Many companion planting effects, both positive and negative, may be the result of allelopathy.
The term is relatively new (hence you won't find it in a lot of books) and has been the matter of some debate, both over definition and whether it is measurable and distinct from other concepts such as resource competition. (see https://en.wikipedia.org/wiki/Allelopathy for more background).