What does this mean if a phosphate leaves ATP to make it become ADP, but now has an i on it?

The little “I” on the phosphate means that it is an inorganic phosphate. It does not mean that it is an intermediate phosphate. There is no such thing as an intermediate phosphate; however, the phosphate can be “between” ATP molecules at the moment, so that would mean it’s simply floating around in the cytoplasm. However, there is only one phosphate that is inorganic in biology, and it is usually denoted with a little “I” if it is not specifically written out as that particular phosphate. It is PO3-/4.

(The “3-/4” is usually the subscript, but this site does not allow for that to be written as such.) The inorganic phosphates, as a group, are usually the ones we mine to get phosphorus for other purposes, such as for the heads of matches in the early 1900s.