At a glance
Connections In Use is the live count of client connections currently established to the MySQL instance, read from theThreads_connectedstatus variable. It is the raw numerator behind pool saturation: how many seats at the table are taken right now, before you compare against themax_connectionsceiling. A steady reading that tracks your traffic shape is healthy; a flat-line at the ceiling, or a slow upward creep that never falls back, is the early signal of a connection leak or an undersized pool.
What it tracks
The card reportsThreads_connected from SHOW GLOBAL STATUS, sampled in real time. That value is every connection currently open, whether actively running a query or sitting idle in a pool. It is distinct from Threads_running (connections actively executing at this instant) and from max_connections (the configured ceiling). Reading it alongside those two tells the whole capacity story: in-use against the ceiling gives headroom, in-use against running tells you how much of the pool is idle. A high in-use count with a low running count is the classic fingerprint of an oversized or leaky application pool holding sessions open without doing work. On managed services the same value appears as the RDS/Aurora DatabaseConnections CloudWatch metric and the Cloud SQL mysql.connections metric.
Reconciling against the source
RunSHOW STATUS LIKE 'Threads_connected'; on the instance for the live figure, or SELECT COUNT(*) FROM information_schema.PROCESSLIST; to see the same connections enumerated; on Amazon RDS / Aurora cross-check the DatabaseConnections CloudWatch metric, and on Cloud SQL the mysql.connections metric.