If the temperature of your shower water doesn't change when someone
flushes the toilet in your bathroom, your shower has a
pressure-balanced valve. This means that when you set the water
temperature, what you're setting is the proportion of hot water to
cold. When someone flushes the toilet, part of the cold water pressure
is diverted to refilling the toilet, so the amount of cold water going
into the shower drops, but the valve automatically makes a
corresponding drop in the amount of hot water so the temperature of the
water coming out of the shower head remains the same.

In contrast, with old-style shower fixtures, when someone flushes the
toilet, the amount of cold water going into the shower drops but the
amount of hot water doesn't, hence the scalding.

Most new construction nowadays uses pressure-balanced valves. It's
even required by code in some places, for safety reasons, i.e., to
prevent the scalding that's made fun of in the commercials.

A step up from pressure-balanced valves is temperature valves. You
tell them what temperature you want the water and they adjust the
proportion of hot to cold automatically to reach that temperature.