The star itself is not rapidly brightening and dimming.
The twinkle is being added much closer to home.
Earth’s atmosphere is constantly moving.
Starlight passes through changing air
Different pockets of air have slightly different temperatures and densities.
As starlight travels through those layers, the light bends by tiny changing amounts.
The path is never perfectly steady.
That changing refraction makes the star seem to shift, brighten, or dim from moment to moment.
Why are stars especially affected?
Stars are so far away that they look like tiny points of light.
Because the apparent source is so small, little changes in the light path can noticeably affect what reaches your eye.
Why do planets usually twinkle less?
A planet may look like a point to your naked eye, but it actually presents a tiny disk.
Light reaches you from several parts of that disk at once.
Atmospheric distortions affecting one part can average against distortions affecting another.
That usually makes planets appear steadier than stars.
Do stars twinkle in space?
Not for the same reason.
Astronauts above Earth’s atmosphere do not have the same moving layers of air between them and the stars.
Remove the atmosphere and you remove the main source of ordinary twinkling.
Is this related to mirages?
Both involve light bending through air with different temperatures and densities.
A hot-road mirage is a much stronger everyday example of atmospheric refraction near the ground.
Star twinkling comes from smaller, constantly changing distortions spread through the atmosphere.
Why do stars near the horizon often twinkle more?
Their light has to travel through more atmosphere before reaching you.
More air means more opportunities for the light path to be disturbed.
So the star is not flickering like a distant light bulb.
Earth’s moving atmosphere is adding the twinkle.
And that’s the short explanation.
Related explanations: Why does a hot road look wet?, Why does a straw look bent in water?, and Why do airplanes leave white trails?