If you mean that this happens when watching videos on-line it is due to a lack of bandwidth - or capability - in your internet connection. Essentially what happens is that the data downloaded is placed in a store called a 'buffer' and what you actually watch is drawn from this. If this gets emptied - because you are watching the video at a a rate greater than the data is coming in - then the video stops while the buffer re-fills and you get the spinning circle to tell you that this is happening.
The only solution is to either improve your broadband connection to an adequate level or to disconnect anything else that might be using the internet so that you are not sharing the available performance with other devices. It is also possible that there maybe a technical fault on your line connection. Always worth having a word with your ISP.