Nothing happens in a strategy until a value comes in from the market. Most of those values come from the indicator library and the price-action nodes, which have their own references. This group covers the two nodes that bring in raw data directly.
Current Price gives you the live price or a bar's open, high, low or close. Data Source brings in series that aren't price at all: macro data such as VIX and the 10-year yield, company fundamentals and earnings, sentiment and dividend yield. Both sit at the left edge of almost every graph.