Hi Siny, long time no see:))
"Hearsay" evidence is often not admissible in court, and there are strict rules about when such evidence will be allowed, which vary from nation to nation. If you've ever watched a legal drama and seen one of the lawyers leap up to say “Objection: that's hearsay,” now you know what he or she was referring to.
"Hearsay" means that which cannot be verified from second or even third hand sources. A classic example of hearsay would be something like “my cousin saw the crime.” In this case, the cousin might well have seen the crime, but unless the cousin can be brought to court to testify, this evidence is treated as hearsay because it has not been verified by the person who actually witnessed the crime.