Subpoena to Court

So ... today, a police officer called me to tell me they were at my work site ... to subpoena me. I asked what it was about and apparently, I "may have evidence" that someone breached probation. When they told me the name, I said, "Who?" I never met the guy. Haha. But, I understand why I was subpoenaed. I just may not be much help. And, I have a legal obligation to provide witness ... or else.