A mnemonic is any method that helps you remember something that you have learned well enough to recognize, but perhaps not yet well enough to recall. Almost all of us frequently see someone we know we have met, but we can't think of her/his name. Almost all of us have sat taking an exam knowing that we know an answer but it just won't come to mind. Almost all of us have had the problem of knowing that one thing is called "X" and the other is called "Y" but we can't keep straight which is which. For these and a wide variety of other potential memory failures, mnemonic techniques can be helpful.
In the above examples, the person, the question, and the object are stimuli calling for learned responses from you. If the response is well learned, you simply give it straightaway. For example, if I ask you when did Columbus discover America, you may be able to answer "1492" immediately and confidently. When you were first studying the history of the world, you may have learned: "In fourteen hundred and ninety two, Columbus sailed the ocean blue." If so, when asked the question, you could recite the little poem to remind you of the date. Thus you were introduced to one of many mnemonic techniques.
In general, a mnemonic mediates between the stimulus and the response. Instead of S -> R, you have S -> mnemonic -> R. It is often the case that the mnemonic does NOT produce the response that is required. It may be just a hint, but enough to prompt your memory of the answer. For example, a woman with a wild hair-do may make you think "hairy" in recalling that her name is "Mary." You may remember that the ordinate of a graph is the vertical axis and the abscissa is the horizontal axis by noticing that your mouth opens up-and-down when you say "ordinate," and it stretches out sideways when you say "abscissa." Thus, a mnemonic helps you produce an answer so that you can recognize it as being correct.
Mnemonics are of NO value for comprehension.
Knowing the names of the axes of a graph does not explain how graphs are constructed or how you should read information from them. You might just call them "vertical" and "horizontal," and still have difficulty when trying to remember which is which unless someone pointed out that the horizon runs flat across the sky. In any event, a mnemonic can help with re calling information but you have to know what the information means.
Like any other skill, using mnemonics is a learned behavior that requires lots of practice. To be sure, anyone can learn the specific examples that are used to illustrate mnemonic techniques. But if you want to include mnemonics in your memory repertoire, you will have to spend a reasonable amount of time learning how to recognize those situations for which mnemonics might be useful, which of the various mnemonic techniques best suits that situation, and then devising some mnemonic that is likely to work for you.
One final point: a mnemonic drops out with repeated use.
After a response is mediated a number of times, it comes to mind directly. At the same time, ou can always fall back on the mnemonic when you have a lapse of memory and the answer doesn't come directly to mind.