Active customers is how many distinct customers you invoiced in a period.
Why count customers rather than jobs
Revenue answers how much. This answers how many, and the two moving differently is the interesting part.
Revenue up, customers flat — you are doing more work for the same people. Good for margin and efficiency, bad for risk, because the business is becoming dependent on fewer relationships. Read it next to customer concentration.
Revenue flat, customers up — you are winning work but job sizes are falling. Sometimes a deliberate move into smaller work, often a pricing drift nobody chose.
Both up — growth.
Both down — the honest version of a slow quarter, and the one worth reacting to early.
The number behind the number
A customer count that stays steady while the names churn is a very different business from one where the same customers return. The count cannot tell you which you are, and the difference is most of what determines whether marketing spend pays back — see lifetime value.
A caution on the period
A customer who buys once a year is not inactive in the eleven months between. For seasonal or annual-service trades, compare the same period year over year rather than against last month, or the number will describe your calendar rather than your business.