A currency strap is like a big paper clip for money. You know how your teacher uses a paper clip to keep all the papers together? Well, banks use currency straps to keep all the money together.
Imagine you have a bunch of five-dollar bills. You would take a currency strap, which is just a long, skinny piece of paper with a sticky part on one end, and you would wrap it around the money so it's all neat and tidy.
But you can't just put all the money in one strap because then it would be too heavy and hard to count. So, the straps are color-coded to show how much money is in them. A blue strap is for $1 bills, a red strap is for $5 bills, a yellow strap is for $10 bills, a green strap is for $20 bills, and so on.
Once the money is all strapped together, bank tellers can easily count and organize it. It keeps everything nice and neat, and makes it easier for them to give out money when people come in to make a withdrawal.
So that's what a currency strap is - just a tool banks use to organize and keep track of all their money!