I don't know how else to state this so you understand it. Bills are written documents that create laws. This bill is a tax bill. It has some verbiage in it that lowers taxes for certain people under certain circumstances.

The bill has language that lays out how long this reduction is to last. The bill then has language stating what will happen when the tax cuts end.

Specifically in this case, when the tax cuts end in X years, taxes increase again by an amount laid out in the bill.

For example, the bill might say something like this:

The tax rate for the money you earn under $100,000 will now be 10% (lets say it used to be 15%). This will last for 10 years. After 10 years, the tax rate for money earned under $100,000 will go to 20% and stay there. This will occur unless some new bill is passed into law that changes it.

The bill literally says that the cuts will last for x amount of time and after that time the new rates will change to a higher rate.

Hopefully this makes sense to you.