What does a service like "Have I Been Pwned" let you find out, and why is that useful?

It tells you whether your email/account has appeared in a known data breach, so you know which passwords to change.

The service aggregates and analyses raw data dumps from breaches — information about millions of leaked accounts — and lets you check whether your address or domain shows up. The breach metadata is public.

Why it's actionable: if your address appears in a breach, any password you used there (and anywhere you reused it) should be considered compromised and changed immediately. It turns "was I affected?" from a guess into a check.

Tip: Use it proactively and set up notifications — haveibeenpwned.com can alert you when your address turns up in future breaches, not just past ones.