Even if you've never heard of the term latchkey kid before you might have in fact grown up as one and just never knew it. Latchkey children were more common back in the days when parents didn't have to worry too much about the welfare of their children when they were away from them because they had to stay late at their job.
Now with there being so many child abductions in the newspaper and the world becoming less safe than it once was the term latchkey child has started to disappear from our everyday vocabulary, which makes it easier to not know what it means. Basically a latchkey kid is a child that returns home to their house from school or from any other activity such as camp or daycare only to come home to an empty house because their parents are still at work.
Either the child in question would have their own spare key to open the door where they live in or there would be a spare key hidden somewhere around the property like in the mailbox or under the welcome mat. The reason why parents would hide the key to their home instead of giving a copy of the key to them is because they were afraid their kid would lose the key and be stuck waiting outside. If you grew up in an environment like the one we just described when you were a kid but didn't know you were a latchkey kid back then you do now. Some parents today still allow their kids to come home to an empty home and trust them to do so on their own but it has become less and less common in today's society.
Even if parents won't be showing up to their house after work for a long period of time after their kids have been let out of school they make some sort of arrangements for there to be somebody at home to let them in and take care of them until they got home; like a babysitter or grandparent. Or they sign their kids up for after school activities like swimming lessons, soccer, gymnastics or art club that will have their kids not only taken care of in a safe environment but also having fun and learning stuff, which might be the biggest factor why latchkey kids are less prevalent than before.