Since the Hub re-broadcasts all incoming information from any computer to all the other computers, the other computers have to know how to ignore information not intended for them. They do this using addressing. The second catch is that only one pair of computers can talk at one time. If two pairs of computers on the network try to talk at the same time, their information gets all jumbled together in a mess of confused ones and zeros. This is called a 'collision'.
Computers have to wait their turn.
The answer to this is that computers take turns at their own speed. They take micro-second turns, which means that two operations can be going on at seemingly the same time, but each only gets half of the time on the network. This is why heavy use on a network can cause slowness of operations. The more computers that are the in the cycle of taking micro-second turns, the less percentage of the time each conversation gets. Before I move on, I'd like to also mention that software can be written so that a computer will listen in on other computers' conversations.