Kotzeir (reaping) is the third of the 11 agricultural labors used in constructing the Mishkan. The melacha of kotzeir involves detaching a growing thing from its source of nourishment. (“A growing thing” is usually plant life or vegetation, but not exclusively.) Harvesting wheat, apple-picking and weeding the front lawn are all examples of kotzeir. Picking mushrooms is also kotzeir, even though mushrooms are fungi and not plants.
Many of the melachos have reciprocal relationships that are equally prohibited: writing and erasing, igniting and extinguishing, sewing and tearing, etc. Kotzeir, however, is not the reciprocal of zorei’ah (planting). For example, if one were to pull down the shade to prevent light from reaching a houseplant in order to impede its growth, that would be an oddly spiteful thing to do to a plant, but it would not violate the melacha of kotzeir. In order to transgress kotzeir, one would have to pluck the plant (or part of it).
Kotzeir is the reason that using a tree is rabbinically prohibited on Shabbos. Accordingly, one may not climb a tree or use something directly suspended from a tree, such as a swing or a hammock. This is also the reason that horseback riding (of all things) is prohibited on Shabbos. It is common for a rider to break off a branch from a nearby tree for use as a switch (see Talmud Beitzah 36b).