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What is the function 'TransformVector' used for?

I am learning the ‘First Person Shooter’ part in tutorials in the official site.
But I can not understand the following line.What is FTransform(CameraRotation).TransformVector(MuzzleOffset) for?
FVector MuzzleLocation = CameraLocation + FTransform(CameraRotation).TransformVector(MuzzleOffset);
Can anyone help me?