The planet Toril is rich in magical energies, moreso than most other crystal spheres. In millennias past, the magical energies were so strong that most lesser beings were spontaneously consumed by the invisible vortices of magic that swirled around them, and those beings who tried to harness the magical energies for themselves were often severely injured as a result.
Eventually the powerful races of the time were compelled to find a safer way of harnessing magic and it’s power, the result of this search was the creation of huge magical field known today as The Weave.
The Weave served as a protective blanket that covered large areas of the planet and insulated everything from the raw magic that suffused the environment. This allowed lesser flora and fauna; such as the trees, animals, and humanoids that dominate the planet today, to flourish. It also allowed intelligent beings that wished to do so, to tap into The Weave and access magic stored within it in the form of spells.
Raw magic is still plentiful on Toril today; although much lower in volume than 30 millennia ago, and The Weave still protects everyone within its confines from exposure to raw magic, but those with the knowledge of how, can bypass The Weave and directly harness raw magical energies to mould into effects of their own design, (and suffer the consequences of doing so).