Noise limit: The noise level limit can be used to stop Corona's progressive rendering once the specified noise level is reached. The noise level limit is available only if adaptivity is enabled.
Pass limit: Stop the rendering after a certain amount of passes is reached.
Time limit: Limit the render time. This is a "security setting" to avoid too long render times per frame if there is an issue in your scene and it takes very long to reach e.g. the specified noise or pass limit.
Noise threshold: Controls how much noise is allowed in the image.
Setting here 0.0 means the entire image is sampled uniformly until either the Max. subdivs value is reached or the Max render time limit is reached.
Max render time: Signals an end to rendering after a certain amount of time has passed.
You can set it to 0.0 to not stop after a certain time.
Min. subdivs – Controls the minimum number of samples that each pixel in the image receives. The actual number of the samples is the square of the subdivs.
Max. subdivs – Controls the maximum number of samples that each pixel in the image receives.