Time Division Multiplexing is the process of dividing up one communication time slot into smaller time slots. We will use the example of a T1 which is time-division multiplexed at the DS1 rate. A T1 consists of 24 channels which are read 8,000 times per second. Each time a channel is read, a value is obtained. Thus, a time slot for a T1 is 1/8,000th of a second. Time-division multiplexing combines the values from all 24 channels on the T1 into the same 1/8,000th of a second.
In the diagram below you can see the 24 channels (CH 1 through CH 24) of a standard T1. At a given point in time (t), every channel provides an input value (not really, but that's a more complicated subject for later). These input values are represented in this diagram by the letters of the alphabet next to each channel in the diagram.
The data passes through the multiplexing device and emerges from the other side as a time-division multiplexed signal that contains the inputs from all the other channels (t+1). Note that all 24 pieces of information are still being transmitted, but are being transmitted together in the same ammount of time as when they were part of their original channels.