ELI5: Why does people say it cold outside and look at the thermometer and it's 51° F

I'm 12 and I was picked up early from school today and I'm going camping and when my English teacher told me my mom was their to pick me up she said be careful it's cold outside. I got in my mom's car and looked at the thermometer on her car and it said 51° F. I was so confused because it cold outside but her thermometer said it was 51°