There are different ways of representing numbers, especially decimal numbers. 3.15×10⁻⁵ is just another way of representing the decimal fraction 0.0000315. Writing a number as a decimal multiplied by a power of 10 is called scientific notation, and it can also be written 3.15e-5 or 3.15E-5. The idea of this notation is to reduce the amount of writing. Numbers of a large magnitude are cumbersome to write and so are numbers of very small magnitude. And it’s easy to make a mistake when writing lots of zeroes in a number, so, to be safe and accurate the “times ten to the power of” notation is used. 0.1 could be written 1×10⁻¹, but that makes it look worse; but 0.000000001 could be written 1×10⁻⁹, 1.0×10⁻⁹ or 1E-9, which definitely is easier to write, because it would be easy to write the wrong number of zeroes if it’s just written as a decimal fraction.
So, when you see 3.15×10⁻⁵ or something similar, you will know that it is not a sum to be worked out, but it’s just a way of writing a number. There are great advantages to this scientific notation when you divide or multiply numbers written like this, because, as you will see in your lessons, the powers can be added together or subtracted from one another, and that makes the arithmetic easier.
I hope this answer helps you understand.