Restar is subtraction, which is removing or taking away a quantity. The document uses examples like having 3 jars and removing 2 jars to show that subtraction leaves 1 jar remaining. It introduces the symbol for subtraction (-) and provides practice problems like having 3 cars minus 2 cars to find how many cars are left. The key idea is that subtraction means taking away and the subtraction symbol indicates to calculate the difference between the numbers.