In the law, testimony is a kind of evidence. It comes from a witness who says that something is true. Testimony may be spoken or written. It is usually made by oath or affirmation under penalty of perjury (which is a crime that somebody can be charged with if they give testimony that is not true).
Some Christians use the term "to testify" or "to give one's testimony" to mean "to tell the story of how one became a Christian".