To try experiments we can work with a local schema definition and local database (as opposed to connecting to a Glean server). If you want to play along with the examples, you can do so as follows:
glean shell --db-root /tmp/glean/db --schema /tmp/glean/schema
> :load /tmp/glean/facts.glean
Now head over to Angle Guide to try some example queries and learn about how the query language works.