An careless person doesn't pay much attention to things or thinks less about consequences. They might forget their keys at home or not check if it's safe to drive during bad weather.
We often use the word "careless" when talking about someone who does something without thinking too much. For example, a careless driver might speed down the highway and cause an accident because they weren't paying attention to the road. A person can also be careless with their belongings, like leaving them unattended or losing track of where they put them. It's not that they mean to hurt anyone, but their lack of attention leads to problems.
Not concerned or worried (about).
