What do they mean by "dual caliber" on this uzi? It says .45 acp & 9mm?

First sentence explains it. two bolts and barrels, you need to swap them to switch calibers. Don't mix them up or you will have serious problems.

"Rare Dual Caliber (45ACP & 9M/M) IMI ISRAEL UZI on one NFA TAX Stamp. Matching Ser#'s on both bolts and barrels"

essentially a UZI with chamber kits for 9mm and .45 ACP, so you can switch out the kits so you can use both cartridges in the same gun by simply switching out a few parts(usually the barrel, bolt and magazine), no need to buy two uzis and makes finding ammo way easier, 2 choices instead of 1.

It's a basic stock with two different bolts and two different barrels you can change out.

it comes with additional parts and barrel so you can disassemble it then reassemble it as the other caliber.

It sounds like if you take the choke out of the gun you can shoot .45 and when the choke is in you can shoot 9mm

You can use either caliber of ammunition...