Well... the weed can indeed be "activated". The basic principle is that the weed contains a lot of a cannabinoid called THCA, which is not active, but when its heated its transformed in THC, which is active, and its the one which makes you high.

The weed can be activated by heating it for some minutes (5-15) at any temperature between 220-300F (but no greater than 330F), or by microwaving it for some minutes (2-3).

If the weed were of low quality, fresh, recently harvested, homegrown, or not properly cured, the increase in its potency is very large.
If it were of high quality or properly cured, the increase in the potency is low or even barely noticeable.

For more info, search "decarboxilation" in this forums... there are several threads about it.