Fact witnesses testify to what they see and hear. They don't offer opinions. There are, of course, a few exceptions. Eyeball witnesses can estimate the speed of a car in an accident or the value of the family home or other owned real estate. For the most part, however, the law confines people to testimony about what they actually saw or heard.
Expert witnesses can express opinions.
Such a witness brings to the stand special knowledge, education and experience relating to a particular case. Experts testify often in personal injury claims, where a plaintiff is trying to prove that an accident resulted in injury. For more than 60 years, admissibility of expert testimony depended on professional acceptance by a group of peers. The same idea applies to others who give expert testimony, such as accountants, engineers, architects, social workers, and people in various lines of business. The old law recognized that there can be lots of ways to skin a scientific cat.