A number in the form a + ib is called as complex number where a and b are natural number and i is called as imaginary unit and i=√-1.  

we can solve the equations like x² - 9 = 0, x² - 4 = 0, etc. But we can not solve the equation like x² + 1 = 0,x² + 4 = 0 etc. because there is no real number whose square is a negative real number.  Euler was the first mathematician who had introduced the symbol i (iota) with the property i² = -1. So by introducing i, now we can solve the equations like x² + 1 = 0, x² + 4 = 0, etc.