Subtraction involves taking away a number from another, which can lead to negative results when the number being subtracted is larger. A number line helps visualize this process, where moving left from zero indicates negative values. For example, subtracting 7 from 5 results in -2, as you move two units left of zero. Young students typically learn subtraction using both fingers and number lines, but understanding negative numbers requires extending the number line beyond zero. The discussion emphasizes the importance of using a number line for clarity in subtraction, especially when dealing with larger numbers.