The world of stock photography is a vast and complex one. There are essentially two strands to it being creative and editorial.
The creative side of stock photography is almost an anything goes area. You can create what you want and what you think any potential buyer might want then upload it to one of the various agencies.
Editorial photography is also vast but is based around images of a specific moment. So it might be documenting a particular weather phenomena or it could be a sporting events such as football, baseball and tennis. Then at the extreme end of the scale it could be events such as wars and disasters.
Selling the images is done either directly, which is difficult in a lot of cases, or through an agency who will represent the work that you produce.
In the past stock photography could be a nice earner for people but with the vast oversaturation of the market and image libraries willing to take practically anything and everything it is becoming a diluted game of chance each month.
So have a look at my thoughts on this particular area of photography and do comment over on YouTube.