In the wiki, it only says:
Catpower is used in a variety of activities. Early on it is used to send out your hunters to bring back rare resources. As the game progresses, catpower becomes useful for many other tasks, such as trading.
This doesn't explain how can I produce catpower, and as I advance in the game I just see my catpower stays at 4.26, and I don't know how to increase it.
Edit: I have edited the wiki with the info in the accepted answer:
Catpower is used in a variety of activities. Early on it is used to send out your hunters to bring back rare resources. As the game progresses, catpower becomes useful for many other tasks, such as trading.
Catpower is produced by assigning kittens to hunter job.