Answer:
SI units refer to the International System of Units. These are the standard units that measurements are typically given in. For example, the SI unit for time is seconds, meters for distance, kg for mass, etc.
Significant figures are the number of notable digits given for a measurement. For example, let's say you measured a piece of metal to be exactly 3.00 meters long. The value 3.00 has three significant figures, where as 3 only has one significant figure. Larger amounts of significant figures indicate a more precise measurement. In this case, by writing 3.00 instead of 3, you're telling me that you measured that piece of metal to be precisely 3.00 meters long.