how to know the meaning of MSE and PSNR value?

i try to use MSE and PSNR built in function and i got this value

mse = 0.1200 peaksnr = 9.2078

and if i not not use im2double i got this

mse = 7.8273e+03

want the meaning of this value ?

 ref=im2double(rgb2gray(imread('dogs.jpg')));
 A = imnoise(ref,'salt & pepper', 0.4);

 err = immse(A, ref);
 peaksnr = psnr(A,ref);