Players are not allowed to “rush” the board.
This is when a player aims to buy 1 of each property color in the first 1, 2 or 3 orbits of the board. In other words, you are not allowed to try to block trading by buying 1 of each color.
The way rushing works, is when you first start the game, you buy up one of each color asap, so that you can then concentrate on getting train stations and utilities, with the goal of long-playing everyone. The aim is to create a situation were trading can never lead to anyone owning 3 properties of the same color, because you own at least 1 of each color and will not ever trade. You then buy up utilities and stations and just go round and round and round and round the board, collecting 200 each time you pass go, with everyone slowly losing 10 or 50 bucks a round, until they are all bankrupt and you get monopoly.
Not sure if that makes sense.
EDIT:
People who do this often will try to trade within the first orbit of the board, it does not matter if you blow all your cash on the first orbit, you will get 200 each time you pass go, and as no one will have houses or hotels, you can build it back up slowly.
You will see people offering silly prices for a card for the first property on the board, or buying up things for too much cash. If you see this, they are probably rushing the board.
Note: I totally made up the term rushing, not sure if it has an official name