In several of the interviews Norton Juster gave to celebrate the fiftieth anniversary of The Phantom Tollbooth, he mentioned one big worry he had: that twenty-first-century readers won't know what tollbooths are (for one example, check out this source).
So, if you've never heard of a tollbooth before, don't worry. You're not alone. A tollbooth is a tiny building on one end of a big highway or road. Before you drive on the highway, you have to stop at the tollbooth and pay a little money. The money is the toll, or little tax, that you pay to get on the road. Nowadays, you're likely to see a tollbooth at one end of a bridge, like San Francisco's Golden Gate.
So, a tollbooth is like a gateway, or a door. (You could compare it to the wardrobe in The Lion, The Witch and the Wardrobe or Platform 9 ¾ in the Harry Potter series. All are gateways that characters pass through to get from their own realities to magic lands.) Once you go through it, you're free to explore new territory. In Milo's case, this territory is the Lands Beyond.
Without the appearance of the tollbooth, Milo wouldn't have been able to get to the Kingdom of Wisdom. Just a reminder to us readers that you can't get somewhere by standing still. You need to break through a passage or a gateway to get your journey started.
(You can read even more about the idea of the tollbooth in "What's Up With the Title?")