The world of stock photography is a vast and complex one. There are essentially two strands to it which are 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 on images of a specific moment. So it might be documenting a particular weather phenomenon or it could be a sporting event such as football, baseball or 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 that will represent the work that you produce.
In the past stock photography could be a nice earner for people but with the vast over-saturation 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 comment over on YouTube.