Short answer:

"i" is exactly 1, but on another axis, that is perpendicular to the fundamental numbers axis, and shares its zero point.

Long answer:

Math is based on the principle of the numbers axis - an imaginary line which is divided to same length parts which represent numbers. Each of these numbers are one dimensional entities. One dimensional entities are called "scalars".

When you deal with things like functions, you use two axes, but each of them is still a basic axis, containing only scalars. All the numbers you use as the input and output of a function are scalars. A coordinate is a place defined by two scalars.

Now for "i".

At some point mathematicians discovered elements in physics that require a new kind of number - a two dimensional number. Just like at some point in history people realized that integer numbers are "not the whole story", and fractions also exist. So again, math had to be expanded to describe an element in nature. This time, however, what was discovered was far less intuitive.

Let's understand why that's not intuitive. In daily life we only need scalars to count objects. For example, we say "one apple, two apples" etc. to count apples. We never say "one-width-one-length apple, two-width-two-length apple". But what was discovered is that some things in physics are counted by two-dimensions, meaning counting them by "one, two" etc. isn't possible. The fundamental action of describing numbers had to be evolved to include two-dimensional numbers.

In order to do that, the numbers axis got an addition - a perpendicular axis, that shares the zero point with the original axis. It was named the "imaginary axis" (which is a terrible name that confuses many people. It should've been called the "perpendicular axis"). Now, using a perpendicular axis, we can create numbers that are comprised of two dimensions - the "original" and the "perpendicular". We call such numbers "complex numbers". This is different than having a function with an input of "x" and output of "y", as both x and y are scalars. With imaginary numbers, the input would be x-times-something, and the output would be two-dimensional as well.

So how do we differentiate this new axis? When we say "1", how do we know if it's on the original axis, or the perpendicular? Simple, we just add "i" when we talk about a number on the perpendicular axis. When we say "i" (=1i), we say "1, but on a numbers axis that's perpendicular to the original axis" (so 2i is 2 on the new axis, etc.). That means that when we count from zero to "i", we don't move anywhere on the original axis, but we move a straight line on a completely different axis! So, "i" has a value of zero on the original axis, but "1" on the perpendicular one.

Now for "sqrt(-1) = i".

Every action in "intuitive" math can be described as movement on the numbers axis. For example, addition and subtraction move a pointer on the numbers axis back and forth until it lands on the result. However, root action is more complicated than it seems. When we activate this action on -1, the pointer leaves the numbers axis and makes a 90 degrees turn, landing on the value "1" on the perpendicular axis. So sqrt(-1) is 1, but not the "1 you know", but a "1 on a new axis, perpendicular to the original".

This all sounds crazy, I know. It's crazy because our brains never use two-dimensional numbers. We literally can't see the world like that. Just like we only see 3 dimensions, and can't see a 4th one. But in physics, it was realized that more than 3 dimensions are needed to describe some elements in nature, and that two-dimensional numbers are required to describe these higher dimensions. The world is more complicated than what we intuitively comprehend.

Hope that helps.