Ok, let's say I'm at 20,000 feet flying west at an indicated airspeed of 100 mph. According to a little conversion calculator I found, my true airspeed is 154 mph. This can vary depending on air temperature. But this mean that I am actually traveling through the air at 154 mph. If there is no wind, then my ground speed is 154 mph. If the wind is blowing west to east at 10 mph, then I have a 10 mph headwind, and my ground speed will be 144 mph, even though my true airspeed is still 154 mph. I am still traveling through the air at 154 mph, but that air is traveling in the opposite direction at 10 mph, so I lose 10 mph in ground speed.

If the wind is blowing east to west at 10 mph, then I have a 10 mph tailwind, and my ground speed will be 164 mph. I am still traveling through the air at 154 mph, but that air is traveling in the same direction I am, so I gain 10 mph in ground speed.

Now let's say my stall speed is 80 mph. Stall speed is indicated air speed, regardless of my true air speed. The less dense air means that I need more of it to provide the lift I need. So if I slow down to 80 mph indicated air speed, I will stall, even though my true air speed is 123 mph.