What is I?

To expand on what anonymous said: the imaginary number is the square root of -1, this is because there is no possible "real" answer to it, but you can use the imaginary number to calculate things (often used in engineering).

$${i} = {\sqrt{-{\mathtt{1}}}}$$

Sometimes you might come across something with a negative surd. To show this in terms of "i", just seperate the square root of -1 from the rest of the surd.

Example:

Let's so I want 7*square-root(-16 * 9):

$${\mathtt{7}}{\mathtt{\,\times\,}}{\sqrt{\left(-{\mathtt{16}}\right){\mathtt{\,\times\,}}{\mathtt{9}}}}$$

$${\mathtt{7}}{\mathtt{\,\times\,}}{\sqrt{-{\mathtt{25}}}}$$

$${\mathtt{7}}{\mathtt{\,\times\,}}{\sqrt{{\mathtt{25}}}}{\mathtt{\,\times\,}}{\sqrt{-{\mathtt{1}}}}$$

$${\mathtt{7}}{\mathtt{\,\times\,}}{\mathtt{5}}{\mathtt{\,\times\,}}{i}$$

$${\mathtt{35}}{i}$$

the i stands for imaganery number. for examply, 2i= 2 imaganery number.