Example json
Some data is made available in part of the flow by an action and now you need to get to an element of this data. The data is all presented in json.
As a simple example of json we can have data like this.{ "level1": { "level2": "data"} }
For a more details description of the json language please have a look at json site.
Query json
Many people would now use the following syntax to query the json to get to the level2 data:
outputs('Compose')?['level1']?['level2']
This would give you:

Optimized query json
A similar query approach, with a slightly different syntax can be:
outputs('Compose')?['level1/level2']
At first I didn’t really see too much of an advantage of using this syntax, but when you reopen the flow you will see the advantage straight away:

Now compare that with the other syntax:

The query on the Compose json actually shows the elements that your flow will try query.
Other approaches
To make sure that your json elements are accessible within your flows as Dynamic content you could also consider using the parse json action. This action I’ve described in the post below.
Use 1 Parse JSON action to validate data
[…] the output generated by the Parse JSON action. If you didn’t use the parse JSON you might have to query json using something like […]