I assume that you're referring to the BBC iPlayer.
To provide a good picture and audio on your screen, the iPlayer attempts to pump an extremely large amount of data through your internet connection. However the actual amount of data that can reach your PC is affected by:
(i) the nominal speed of the connection that you've paid for ;
(ii) the amount of concurrent internet usage of other people in your street (who share the 'main feed' into that street) ; and
(iii) any policy that your ISP might have on 'throttling' data using certain services (so that everyone in your street can get a fair share of the available internet access).
So there can be times when the iPlayer is trying to send data to your PC at a rate faster than your connection can cope with. That results in iPlayer having to pause while it waits for the next block of data to arrive.
The BBC is aware of this problem, so it offers a 'low bandwidth' solution. By selecting that option the iPlayer will send less data to your PC. That (hopefully) will mean that data can continue to arrive in time for the picture to be displayed without any annoying pauses. The downside is that this can only be done by sending a lower quality signal, so the picture quality won't be as good as the 'full' version.
Unfortunately the technology behind the BBC iPlayer is ahead of the technology used in telephone cables and exchanges. Most people in the UK have (nominal) internet access speeds of around 2MBps or 4MBps. For everyone to be able to use the BBC iPlayer without any problems, speeds of around 100MBps are really needed. These are already commonplace in technologically advanced countries (such as South Korea) but, because of the massive costs involved in replacing every phone line in the country, are unlikely to be seen in the UK for many years. (One estimate for the UK to reach South Korea's current position is around the year 2035).