Count is the simplest of aggregate functions, and will just return the count of matching entities.
Examples
Count all records in Enterprise Tester
Count
Count all records in a project
Count WHERE Project = 'Project X'
Count all incidents in a project with a status of "Open"
COUNT WHERE Project = 'Project X' AND EntityType = Incident AND Status = Open
Count all incidents, grouped by their status
GROUP BY Status { COUNT } WHERE Project = 'Project X' AND EntityType = Incident