Broody is when they decide they are actually going to SIT on their eggs until they hatch.
I have never had a broody as mine are all still chicks... but this is what I have learned on the board....
When a chicken goes broody she will either lay until she has a full clutch of eggs... around 9 or so and will then sit on them constantly, maybe only getting up once or twice a day just long enough to get a drink and stretch her legs.
A hen can go broody on someone else's eggs.
You can sometimes switch out the eggs for eggs you want her to hatch or sometimes with day old chicks.... done by placing new egg or chick in hand and reaching under her while she is sitting and performing a slight of hand switch-a-roo.
A broody hen will often growl, peck, defend and act all grouchy about her clutch of eggs.
Some breeds are better broodies or more likely to go broody... silkies, bantams, etc. Some have had the broody gene bred out of them... leghorns, etc.
Make sure to have water and food available to the broody so she can reach it from where she is.
Sometimes you can move a broody to another spot. Most reccomend you seperate her from the rest of the flock as they may pick on her or run her off of her eggs. Sometimes if you move a broody, she will abandon the eggs. Having movable boxes that can just be picked up, hen eggs and all and moved to a new spot is very useful.
Broody poop is the WORST! They spend so much time on their eggs that their first poop after they hatch can be startling and scary, especially to the first-time chicken owner.
That is what I have learned so far