Hey there! Hello!
For this question, I believe the answer will be something along the lines of a passcode/code lock feature. This basically prevents anyone without knowledge of your device's code access to your device, which is especially handy in case you lose it and don't want other to have free reign of your information.
Without it, yeah, anyone can simply swipe into your device (or, press the home button or whatever they did with iOS 10) and use/view information on it.
If you have multiple choice answer and my answer doesn't seem to fit into any of them, let me know and we'll take it from there.
Hope this helped you out! Feel free to ask me any additional questions if you have any. :-)
