An expert is someone who is particularly knowledgeable or good at something specific. No one is born an expert. An expert may have studied at a university, worked in a profession for a long time or read a lot about a subject. That's why it's easy to talk to them about it, ask them questions and learn from them.
There are often no fixed rules or even laws about when you can call yourself an expert. When you get a degree at university, you get a diploma. That says that you know a lot in that field. But it can also be that someone who has not studied knows a lot.
The word expert originally comes from the French language. The adjective "expert" means something like "experienced" or "knowledgeable". We have been using the term since about the 19th century. The opposite of an expert is a layman. This is someone who knows just as much or just as little as other people.
Experts are, for example, scientists: they do research and thus make sure that humanity knows more about something. Other experts teach people something, such as teachers. Or you need expert knowledge to be able to make something, like a craftsman or engineer, or to cook, for example. Finally, you need to know a lot if you want to write novels or paint pictures as an artist. Painters, for example, need to know how colours work and how to mix colours.