Starlight reaching a ground-based telescope has passed through moving layers of air. Those layers bend the wavefront unevenly, so the image blurs and twinkles even if the telescope’s own mirrors are excellent.
A moving atmosphere
An adaptive-optics system watches a reference star close to the target. When a suitable natural star is unavailable, a laser guide star can provide a reference in the upper atmosphere. A wavefront sensor estimates the distortion and a computer commands a deformable mirror to change shape rapidly, countering part of the atmospheric error.
A mirror that responds
The correction depends on how close the reference is to the target, the speed of the atmosphere, wavelength, and system design. It does not make the atmosphere disappear or replace the advantage of a larger aperture. It is a feedback loop that improves the useful detail in selected observations.
Related: why a telescope’s aperture controls resolution.