Well... actually there is some THC in fresh weed, but the amount of it is GREATLY increased when the weed is properly cured.

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Frequently asked questions
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In the plant the cannabinoids exist mainly in their carboxylic forms as cannabinoid acids. However, the phenolic form of THC is responsible for the psychotropic and the most medicinal effects. Decarboxylation (separation of CO2) to the phenolic form occurs readily over time, upon heating or under alkaline conditions.
The ratio of THC acids (THCA) to phenolic THC has been reported to range between 2:1 (Africa) and >20:1 (Switzerland) in leaves and flowers of Cannabis sativa. In plants grown in Middle Europe (United Kingdom) from Moroccan, Sri Lankan and Zambian seed stocks the THCA/THC ratio was 17:1 compared with 2:1 in the plants from the original areas (Africa, Asia). In hashish (cannabis resin) the THCA/THC ratio was reported to range between 6.1:1 and 0.5:1, the latter in hashish from India. Thus, the percentage of phenolic THC of all THC in cannabis products varied between less than 5% in cannabis leaves grown in Switzerland up to 65% in hashish from India.
Cannabis products with a high content of phenolic THC (e.g. hashish) may be very potent without heating, but usually the potency of cannabis products is considerably increased with heating (smoking, cooking).
See, if "The ratio of THC acids (THCA) to phenolic THC has been reported to range between 2:1 (Africa) and >20:1 (Switzerland)", it means that for one part of THC, the plant may contain from 2 to more than 20 parts of THCA.
It means that when you transform all this THCA in THC, through decarboxilation (curing), the potency of the weed can be increased up to 20 times.