I think the easiest way to explain JSON Schema is to look at an example snippet of JSON and the corresponding JSON Schema. So, I created an image that shows both and is hopefully self explanatory:
(Click to enlarge, if you are on a mobile device)
A JSON Schema validator can be used to validate a JSON document against a JSON Schema. This can be done online (e.g. https://www.jsonschemavalidator.net/) or using a library of your favourite programming language. In the implementations section of json-schema.org you can find a couple of libraries to work with JSON Schema.
In case you want to copy/paste some of the sections of the image: You can find the example as text on GitHub.