Very big and very small numbers are hard to write down because of all the zeroes and they're also hard to quickly evaluate and compare.
Instead of using a long sequence of decimal digits to represent numbers, scientific notation uses a shorter number multiplied by a power of 10 and it's always in that form and if it's not, it's not in proper scientific notation.
Scientific notation uses positive exponents to represent very large numbers and negative exponents to represent very small numbers.
I have attached an image which shows what scientific can be used for and an example of a number written in scientific notation.
