broadcast [ v]

What is broadcasting? And what happens when you do it? I see that many scratchers use this block in their projects but I am never able to understand how to use the block and what it does.

It allows one script/event to trigger additional scripts to work.

So a sprite hitting the edge of the screen could broadcast so the Stage gets a message to switch backdrop. A scoring function could broadcast when a certain score is reached to get multiple sprites to do things. For example, switching to the next level may mean sprites appear/hide, backdrops change and so on.

Thanks for the explanation! But how would I know what the message does? And what does creating a new message do?

Try using it (because I don't really know how to explain it )
Just take out two sprites.
Sprite one:

when green flag clicked
say [Hello!] for (2) secs
say [What is your name?] for (2) secs
broadcast [Your name]

Sprite two:

when I receive [Your name]
say [My name is (blah)] for (2) secs

This block basically makes two sprites communicate with each other and tell each other to do something.

You need to distinguish between messages so you give them names.

broadcast [next level v]
broadcast [player1-turn v] and wait   // don't continue this script until all receiver scripts complte

or whatever. Creating a new message does nothing untiil you write at least one receiver and tell the project to make the broadcast.

when I receive [player1-turn v]
code goes her for player 1 to take a turn

It brodcasts a message across the workspace. Like this:

when green flag clicked
broadcast [Cool v]

when I receive [Cool v]
move (10) steps

Between different sprites and the backdrop.

Check out the Scratch Wiki, tons of great nfo in there