When a number is expressed with exponents, or one number to a power of another, it is considered to be in index form. For example, 27 can be written in index form as 3^3. This is because 27 is 3x3x3 or 3^3.
A common question in math will be to write a number in index form using a different number as base. An example is being asked to write the number 16 in index form using base 2. Because 16 is 2x8, and 8 is 2x4, and 4 is 2x2, 16 in index form using base 2 would be written as 2^4.