Numbers can express how much there is of a certain object. The objects can exist in the mind or in reality. You can have 1 marble, 35 marbles, and even wish for an infinite amount of them. You can have 33,3452 marbles and you can have $\sqrt 2$ marbles. You can even have a minus number of marbles. A bit more abstract but I can imagine that should Ì get them in reality I have to give them to someone else.The negative number is always related to the real positive number. A negative charge exists in comparison to the positive charge only. Likewise, a negative temperature can only be defined wrt the real positive temperature. So negative numbers are numbers that can only be defined in the context of poitive quantities. On their own they have no meaning. They are the result from taking the opposite of real positive numbers. Positive numbers are real in the sense that they represent a true quantity. You can have one apple for real but minus one only in the mind. A charge of 5 exists as well as a charge of -5 but these numbers don't point to a really existing thing which you can have a number of. The plus and minus are merely used to point at opposite effects they can have. A velocity can be plus or minus but you can't have -6 of them. So a negative number (if used when expressing how much of a thing you actually can have) is imaginary too.
So this means you can have -5 marbĺes but this merely means that you must give them away when you should get a hold on 5 real marbles.
But what about i marbles. What mental operation do we have to perform on the marbles to say you have 5i of them? If -5 apples just is the operation of taking the negative (meaning you owe) to what corresponds multiplicating by i (instead of -1)? If I have -5 marbles I think of giving back 5 marbles as soon I get them. But what should I think when I have 5i marbles?
Will the marbles themselves change (contrary to taking the negative)? Is there something I have to do with them? Insread of giving them away, maybe rotate them? So if I have -5i (sqrt-25) marbles I have to rotate all five and then give them away? Or what? Why is it called imaginary?