When you enter a number and follow all the instructions, you always get 5 at the end.
The idea is that, if (this is an example) 3 was added at the start to another random number, the number and 3 seem to be added together, but by subtracting the other number at the end, the answer will always be 3.
If you look at the blue line and orange dots that are next to the instructions that the interaction gives, the blue line (a random number) and the orange dots (the units that the interactive tells a person to add) are separate, even though when the instructions are used, it is one number that a person sees.
While the instructions are performed, the answer from the second last step is (random number+5). You can see that even though the number goes through a lot of equations, you still end up with (random number+5). The next step subtracts the random number, leaving you with 5. Whatever number is chosen, the answer will always be 5.