line of sight mean that there is no obstacle limiting light going from one to another. For example if they are in separate rooms. The IR communication works like your TV remote, you have to have line of sight between remote and TV for remote to be able to send signal to TV what to do. Here is the same thing, if you have the line of sight you can use IR for them to talk to each other. If you do not have line of sight you have to go with RF and that require transmit and receive circuits that are not easy if you want them not to drift too much... you can get some failry cheap transmitter modules with low quality, slow speed and low range but they will work, or you can go more expensive or even digital with BlueTooth or Zigbee...
I suggest you first try simple communication with IR as explained in Nigel's tutorials. If you do not have much experience, I suggest you first go trough all Nigel's tutorials before you start your project.