Depends. Some of them use the word as a catch-all term for parents. Others use that word specifically for people that think having children is literally the only important thing in the world and base their opinions and behavior on that.

Behavior and opinions such as: needing to belittle childfree people and "debate" their life choices, having unearned sense of superiority because they happen to be parent, thinking everyone must sacrifice their time and resources for parents and their children. Stuff like that. You don't even need to be parent in that case, incel with very strong opinion on how woman's purpose is to be a mother is also a breeder, even if he doesn't have any children himself.