That means a skill. You can go to school-college/university for it but usually not very long. 

So a skill in America could be plumbing, heating/AC, construction, running equipment, etc. 

the better you are at your skill-the more $ you make. Usually. 

Sometimes it's people's true passion in life. So art could be a skill. Like a "skilled" painter 

to learn a trade = to acquire a skill in/ to gain knowledge of something

As I understand "trade" originally meant "a path" or "a track" or "a course". Therefore, it means "take a path in your life".

Also, later the same thing means "any occupation pursued as livelihood" so, in short, it means "learn something with which you can make a living".