Sentry’s library wraps your code in a big catch block that captures exceptions that occur when people use your app, website, plugin. It then sends them off to their servers for reporting.
As a developer, you get a notification that someone received an error with additional context on what the user was doing and what the error looked like. This gives you a hint on what you might need to change to fix problems users are experiencing.
There are many services like sentry out there, Rollbar, Bugsnag, Raygun are some popular alternatives.