A dye pack is a small device that can be hidden in a stack of money or attached to a bag of cash. It's used by banks and businesses to deter robberies or identify the perpetrators of a robbery.
When a thief tries to steal the money, they won't know that the dye pack is there. Once the bank teller activates the pack during the robbery, the thief typically has around 20 seconds before the pack explodes, releasing a bright, permanent dye. The dye covers the money and the thief in the immediate vicinity, making the stolen cash unusable and easier for police to identify.
So basically, a dye pack is like a secret bomb that banks use to protect their money. If someone tries to steal it, the dye pack explodes and turns the thief and the stolen money bright and colorful so that the police can find them easily.