In solving quadratic equations, we often get a square root of negative number as part of values of roots.
For eg x2+ 1= 0 gives x = sqrt of (-1)
The extraction of square root is not possible as no real number exists which when squared will yield a negative number (-1, in the above case).
So, a new type of number known as imaginary number is used to extract the square root of a real number.
Eg sqrt (-4) = 2 sqrt (-1)
So, the square root of any negative number can be expressed in terms of sqrt (-1), known as imaginary number and is denoted by i (iota).
You will study about these in your higher classes.