Insights counts things over time. It is the report you use most.
What it answers
How often an event happens, whether that is rising or falling, and how it differs between groups.
Breakdowns
Split any measure by a property — plan, country, referrer, device. A flat total that turns out to be one segment growing while another shrinks is the usual reason to break down.
Time
Choose the range and the granularity. Be careful comparing a partial period against complete ones; a dip at the right edge is usually today not being over.
From here
A number that looks wrong is usually best chased into Users, to see individuals rather than aggregates.