Assuming A2 contains sheetname Truck_1, A3:Truck_2, A4:Truck_3 and so on.
If in B2 we type: =A2&"!A1" => displays "Truck_1!A1", it is a text but reference.
INDIRECT(Text) is to tell Excel that Text is reference.
INDIRECT(A2&"!A1")="xyz" if cell A1 of sheet Truck_1 contains "xyz"
Copy INDIRECT(A2&"!A1") down we have INDIRECT(A3&"!A1")=Truck_2!A1 reference automatically.
So, INDIRECT helps to get sheetname in A column to avoid re-typing in each row.

Back to your wookbook, you have 18 sub-sheets, then you have list of sheetname from A2 to A19 which is Truck_1, Truck_2,..., Truck_18, then drag the formula in B2 to B19.

Hope it is clear for you.