24 Hour price change Mean If a currency price is showing ABC per coin and its price % is +X% or -X% then this reference is taken from which last price i.e. which point of time the price is taken
The 24h % change is the difference between the current price and the price 24 hours ago. 

The current price of bitcoin is $14988 and was $14937 24h ago. So the 24h change is +0.3%, even if there was a huge spike between this period of time.