A json file is simply a way to represent data, much like a txt of html file. But the important part is that is has rules to it that can be translated to python objects.
The json file requires an opening {} and at least one tag to be useful, you can read the full documentation here
An example would be this
{ "name":"John", "age":30, "city":"New York"}
and using the json built-in library you can read it effortlessly
import json
# some JSON:
x = '{ "name":"John", "age":30, "city":"New York"}'
# parse x:
y = json.loads(x)
# the result is a Python dictionary:
print(y["age"])
As to why it is important, it is one of the best ways to send data through the internet since it is supported by the HTTP standard and thus is of great use in building APIs, as well you can use it to store information to be saved when the program stops running.