Concerning means causing worry or unease about a situation. When a teacher notices a student's grades dropping, that pattern is concerning because it might signal a problem. When a doctor finds something concerning in test results, it means something needs attention or monitoring.

A few mosquito bites aren't concerning, but dozens of them might be. A single missed homework assignment isn't concerning, but missing several in a row could be. Something concerning makes you pause and think, “I should probably keep an eye on this.” Concerning sits between “no big deal” and “emergency.” It's not cause for panic, but it's not casual either.

Concerning can also mean “relating to” or “about.” You could say, “I have some questions concerning our upcoming field trip.”