A split break is when you take your 45 minute break in two parts, the first at least 15 minutes and the second at least 30 minutes.
If you mean split rest it is when you have a period of at least 3 continuous hours rest during the shift and at least 9 at the end of the shift. This doesn’t count as a reduced daily rest so you can do this even if you have used all your available reduced daily rest periods and you could do it 6 times between weekly rest periods. The usual rules about the rest being completed within the 24-hour period from the start of your shift apply.