Identifying your Users table
Head to the Query Builder tab in Trevor.io and locate the table that contains your users.
It's probably called something like customers, or users, or perhaps the profession your users belong to - e.g. teachers / photographers / doctors etc.
Click to open it.
Getting to know your Users table
In theory, each row in this table represents a user.
Each column represents some associated characteristic, like:
ID
name
email
country
sign up date
address
sign up source etc.
Finding connected data
If you see a drill-down button to the left of each row, it means your Users table is connected to other tables (e.g. the table containing transactions / orders / sales).

Click the drilldown buttons to explore the data connected to each individual customer. For example, in the GIF above, you can see the transactions made by the customer with ID 1.