Let's say we have a post object in our schema. A post can have an array of
references to tags telling you what topics the post covers. Each tag has a
slug and we want to get the string value for each slug.
Let's say we are interested in the post with _id of 123.
Here is how we can achieve that with a groq query:
*[
_type == 'post' && _id == 123
]{
'tags': tags[]->slug.current
}.tags
=> ["javascript", "react-js"]
If the schema was such that each post just had a single tag reference, then
you could write the chain of references as tag->slug.current. Because it is
an array of references, we need the [] to declare that we want each value.
The -> operator follows the reference. Otherwise we'd just have access to the
_ref and _type values.
The final .tags unnests the tags value we gathered into an object. Then the
result is just the array of slug values.