The Lagrangian points are indeed points, infinitesimal in size, where the gravitational forces from a planet and another body (generally the Sun or a moon) exactly balance the centrifugal force. But, as you guessed, surrounding each actual La-grangian point is an extended zone where a spacecraft can conveniently park itself in an orbit that requires little fuel to maintain. When you hear about a spacecraft orbiting at a certain Lagrangian point, it really means the probe is traveling within or near one of these extended, three-dimensional islands of orbits.
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