Hey there!
The value of 15 tens would be 15 tens added together. Instead of adding 15 to itself 15 times, the process can be shortened by using multiplication. Multiplication basically takes a number (10) and adds it together a specified amount of times (15) with only one step.
15 × 10 = 150
By the way, when multiplying something by 10, you can simply add a zero onto the end of the other number.
Hope this helped you out! :-)