Above it says "One of the COMPRESSION constants" for the parameters. For any of you that found this confusing, you can find a list of the constants here:
As an example for working with a jpeg image:
$image = 'jpeg_image.jpg';
$comression_type = Imagick::COMPRESSION_JPEG;
$im = new Imagick($image);
This class does not actually compress the images but rather sets the compression type that is going to be used.
If you would like to see how to compress the image see: