What do you call it? Not quite the back door, but it connects to the garage. Garage door? No that's confusing. The side door? In any case, whether or not you have a name for it, that door is important!
If you have a door that connects between a garage and the main part of your home, it needs to be secured properly. This is because the garage door proper (the car sized one!) is rarely very strong and thieves breaking through it can take an easy walk into your home if the connecting door suffers from sub-standard locking.
Have a look at your locks. Are they BS3621 with the heart-shaped kite-mark on it? What do your insurance documents say?
Don't leave any weak points!