Your fancy new fangled camera meters for middle gray (18%). My old vintage cams meter for middle gray (if they even have a meter).
So when you try to take a pic of a white cat (or snow, or anything white), the meter tells you the settings for making the cat gray. You need to add two stops of exposure to get white.
When you are taking a pic of a black cat, your meter tells you the setting to make the cat gray. You need to subtract two stops of exposure to get black.
If you are shooting a gray cat, then the camera is giving you the right settings for gray, but you could make the cat white or black by adding or subtracting two stops.
You can get a middle gray card at any camera shop/online for $5. If you don't want to carry it around, then compare the difference according to your meter with the gray card and the palm of your hand. You'll probably find that your palm is about one stop brighter than the gray card. In the field you can meter off your hand. Also a healthy, green grass lawn is approximately middle gray tonally.
A little more complicated, but along the same lines as using a gray card is to use anything black (boots, camera bag, pants, coat, etc....) To get to middle gray after metering off anything black add two stops. This is how I do it in troublesome lighting conditions. It seems easier to find black objects to meter than middle gray.
If you are using auto-exposure modes then you may have to figure out how to use your meter-lock, or when you recompose the snow scene the meter will just take a new reading and reset the camera. It may have it's own button, or holding the shutter button down 1/2 way may lock the meter. Read your manual.