A transparent latch is a device with a data input and a control input. The control input has two states which may be called "track" and "hold". Some devices will regard a "high" on the control input as "track" and a low input as "hold"; others do the opposite. Whenever the control input is in the "track" state, the state of the output will continuously attempt to follow the state of the data input (there will a short delay between the time the data input changes and the output reflects the change). If the control input goes from the "track" state to the "hold" state, provided that the last change to the data input has had a chance to reach the output, the output will hold its value until such time as the control input goes back to the "track" state.
While transparent latches may be used in many ways, it's important to understand at least two usage scenarios. In one scenario, the latch is used to turn a signal that will sometimes hold valid data and sometimes hold invalid data, into a signal which will always hold valid data. This is done by keeping the latch in the "hold" state any time the data input might not match the desired output data. To change the latched data, one would put the desired data on the input, then set the latch briefly to "track" state and back to "hold" state, being careful that the data input does not change to an unwanted value while the "hold" signal is active. This arrangement could be used to e.g. control 64 outputs using eight control signals and eight data signals. Each control signal operates eight latches, one of which is wired to each of the eight data signals. One could use edge-triggered flip flops just as easily as latches, but the circuitry for a latch is somewhat simpler. Note that an edge-triggered flip flop in this scenario would ideally trigger on the transition from "hold" to "track".
In the second usage scenario, the input may not be meaningful at the time the latch switches to "transparent", but will become meaningful prior to the latch switching to "hold". If the devices that use its output won't care about its state until some time after the latch has switched to "hold", then it will be the state of the data input at that time which will be fed to the output. One may be able to use an edge-triggered flip flop in this scenario, but it must trigger on the transition from "track" to "hold". Note that if the data input to the latch becomes valid a significant time before the transition from "hold" to "track", the output will do likewise. By contrast, the output of a flip flop would only become valid when the clock changed.