It simply means that the information required to display the image is being 'buffered' or, to put it another way, your PC isn't receiving the information quickly enough (or alternatively, it can't process it quickly enough). These are the most likely causes:
1. Your PC isn't 'up to the job', in terms of its processor speed and memory capacity. With most modern computers this is unlikely but it might be worth shutting down any unnecessary programs to see if the iPlayer performance improves. (Press Ctrl+Alt+Delete and go into the Task Manager. Stop all background programs unless you're sure that they're strictly necessary);
2. Insufficient download speed. This is the most likely cause. If you've not got the fastest available connection offered by your ISP, you might need to upgrade. If you've already got a fast broadband connection, complain to your ISP. (Remember though that you effectively share a connection with others on your street. If all of you neighbours are downloading files from Limewire simultaneously, you connection speed can drop dramatically).
3. Insufficient capacity on the BBC server. It's a possibility but unlikely, simply because the BBC has the largest website in the world (except for those which are user-generated, such as Youtube) with the technical capacity to match it. Additionally, if this was the problem, all iPlayer users would simultaneously experience problems, which does not seem to be happening.
4. Slow 'backbone' speeds on the internet. At certain times of day, the internet slows down significantly because of increased usage. These are usually around the times that UK kids get home from school and then again when US kids get home from school. Try to avoid these times of day when a fast connection is important to you (such as when watching videos).
However, I repeat that the most likely problem is the speed at which your ISP sends the data to your