I believe it means the imaginary number. It could however just be any number as x or n or y.
Imaginary number is what you get when you take the square root of negative numbers since no actual number when squared gives a negative result, people made up a number and the name imaginary numbers sticked to it. I is the imaginary unit number but could also stand for what I believe is the unit matrix which has 1's across the diagonal and 0's everywhere else.
I assume it's imaginary number that the context is meaning. 8i could be the square root of -64
i^2 = -1
This makes some mathematics easier, even though we don't actually know what i itself is, it is unreal.
For example: What is the square root of -16?
The answer would be 4i since sqrt(-16)=sqrt(-1) * sqrt(16)
Which therefore gives us i(since i^2 is -1) and 4(because sqrt(16) is 4)
Then we end up with i * 4 which can just be written as 4i.
Hope I helped!
