For a good explanation read this post from EFF.
Examples (depending on the service/company/government):
They know you rang a phone sex service at 2:24 am and spoke for 18 minutes. But they don’t know what you talked about.
They know you called the suicide prevention hotline from the Golden Gate Bridge. But the topic of the call remains a secret.
They know you spoke with an HIV testing service, then your doctor, then your health insurance company in the same hour. But they don’t know what was discussed.