The word amateur is normally used to mean: somebody who does something (e.g. a job) without being paid for it, or without having had training or education for it.
The word “amateur” comes from a French word meaning: "lover of". An amateur is someone who does something (e.g. play the piano, football) because they enjoy doing it. They are not doing it in order to be paid. It is not how they earn money to live.
People often forget the real meaning of the word “amateur”. It is often used to mean: someone who is not good at something. Someone’s performance might be described as “amateurish”, i.e. not very good. On the other hand, they might be described as having done a very “professional” job, i.e. a very good job.
Although one might expect a professional to be better than an amateur, this is not always the case. Someone might be a very good amateur pianist, possibly better than some professionals.
In many sports (e.g. tennis or football), amateurs and professionals used not to be allowed to compete against one another. This was not always because amateurs were thought to be not as good as professionals. In some countries, such as Great Britain in Victorian times, this used to be because of social class distinction. Amateur cricketers were rich gentlemen from the upper classes who did not want to associate with people from lower classes. This attitude took a long time to change during the 20th century. Until the 1970s most Olympic events were for amateurs. Athletes were disqualified if they received money for the event.
Amateur boxing requires mandatory head & nose gear, unlike in professional kick boxing & boxing, the head is unprotected.
In some jobs it would be impossible for amateurs to be allowed to work. One cannot have amateur doctors or lawyers. In some areas, however, amateurs can be very useful, e.g. in computer programming or astronomy or birdwatching.