Indirect ("sheet2! A2 ") represents the A2 cell of the sheet2 table, but it is not usually used in this way. In this case, you can write sheet2 directly! A2 will be fine.

If your page is signed in the cell A 1 and you want to quote the cell A3 labeled A 1, you can write it as indirect (A1&; "! a3”)

Supplement to your question: I can't explain this, this is its grammatical rules, tag name! Cells, you can see these rules for yourself. For example, if you enter an = in the SHEET 1 table and click a cell in SHEET2 with the mouse, you will know how it is represented in the formula bar, and you can also view it in this way across workbooks.

Example: if you want to calculate the sum of cells in SHEET 1 table A 1:A 10, you can use the formula = sum (SHEET2! A 1:A 10)