IDS
In plain terms
A system that detects and alerts on suspicious network activity.
Going deeper
An Intrusion Detection System monitors traffic or hosts for signs of attacks and raises alerts, but doesn't block on its own. It's a watchdog.
Under the hood
Signature- or anomaly-based; detection only, unlike an IPS which can also prevent.
Examples
Snort, network monitoring, threat alerting, anomaly detection.
Think of it like...
Like a security camera that alerts guards but doesn't lock the doors itself.