This will mainly be when you are leaving a minor road to join a main road to turn left, right or possibly needing to cross a main road ahead (at crossroads). Where the junction is not light controlled you will have either 'Give Way' markings or a 'Stop' sign.
Look out for advance warning signs. Where there is a Give Way sign you must give way to traffic on the main road. If on approach you can clearly see it is safe to emerge then you can do so without stopping. (If in doubt, don't pull out!) Where there is a Stop sign, you must stop at the line.