What does it mean when the check charging system indicator comes on?

it could mean the the alternater is not charging anymore.qick way to no is to start truck ten remove the positive from batery if truck stops then it is not charging and you are running on your battery.

It means to 'have the charging system checked'. Watch with a scan tool for proper command if PCM controled , or with a ampmeter or other suitable testing device. May still be charging at full voltage , just reduced current or voltage, for enough time to trigger the light. Voltmeter may not be accurate.