Broadcasting from iMac to bluetooth spekerg

I haven't yet received the bluetooth speaker that I bought upon learning that It would be easy for it to receive songs from my iMac's iTunes library. However, I now realize that I must need something else. What is it? How will the speaker "know" which song is playing? Will the song start playing on the speaker at the same time it plays on my iMac? What if I want to listen to something else? There must be something between the computer and the computer. What is it?