Can someone please explain the concept of out-of-the-box components.
I have some doubt about your question. OOTB (out-of-the-box) is term used to refer a fuctionality that you get buying the product. In the AEM, this is the components that you can use to build a form like fields. You can extend this OOTB components adding new functionalities to them. One simple sample can be a component for ZIP-CODE that get the zip number and search a adress detail in a external source and fill other fields like road name, state...
I hope that this can help you.
Component/features that is provided with the product as you bring the server up and are ready to be used as is are termed as OOTB component.
Offcourse these feature can be extended further and can be combined with each other to achieve complex scenarios.
For example, if you create an adaptive form you get text field component, date picker, drop-down component and so on; these can be used as is. You can also create your own component by extending OOTB component.
Agree with both @Jean-foxfly and @Mayank