The aspect ratio is how square or squashed an image is. It's often useful to crop a picture to be an exact shape, for example, so that it fits exactly into a 6 by 4 inch print.
A 6x4 print has the aspect ratio of 3:2. So to crop an image to fit exactly on a 6x4 print, select the Lock aspect check box and the 3:2 option. Now when you drag the crop rectangle it will be exactly the right shape for the paper.
Similarly computer screens are usually 4:3 aspect ratio. So if you want to create a picture that fits exactly to most computer or TV screen so there is no gap above or to the side, select the 4:3 aspect ratio.
If you select the 'current' aspect ratio then the crop rectangle will be exactly the same shape as the original picture.
Un-check the lock aspect button allow any shape crop region.