The "Log" function on a graphing or scientific calculator is a key that allows you to work with logarithms. Logarithms are ways to figure out what exponents you need to multiply into a specific number. For example, using the "Log" function on the number 10 would reveal that you have to multiply your base number of 10 by itself one time to equal the number 10. The log function on all calculators works essentially the same way.
Type the number you're working with into your graphing or scientific calculator. For example, type "1000."
Press the "Log" button on your calculator. The number you immediately see is the exponent for the original number you entered. Assuming the base number is 10 (which it will always be on a graphing or scientific calculator), you have to multiply 10 by itself the number of times you see onscreen to reach your original number.
Check your work. Entering "1000" into your calculator and pressing "Log" will result in the number "3" appearing on screen. This means that you have to multiply your base number of 10 three times to reach "1000." To check your work, perform the math long-form: 10 x 10 x 10 = 1000, which was your original number.