A very basic definition is that chunking is a way of organizing information into familiar groupings. This is done with all sorts of information, including numbers, single words, and multiple-word phrases which are collapsed into a single word, to create acronyms. The main advantage of this type of mnemonic device is that it enhances retention and memory.
Example: How do you remember the names of the 5 Great Lakes? If you just remember the acronym, HOMES, you may find it easier to remember that the names of the Great Lakes are Huron, Ontario, Michigan, Erie, and Superior.
A Retrieval Cue is a prompt that help us remember. When we make a new memory, we include certain information about the situation that act as triggers to access the memory.
For example, when someone is introduced to us at a party, we don't only store the name and appearance of the new acquaintance in our memory. We also include external cues about the situtation like what kind of party it was, who made the introduction, what cocktails were served, or what music was playing. We also include internal cues like what mood you were in at the time, or what you thought of the person being introduced. When we try to recall the person, having one or more of these cues present will help us remember better. So when you meet the person again, it would be easier to remember them if you bumped into them at another party, or you saw them with the same person who introduced you, or you were in the same mood as when you first met them.
Retrieval cues act as triggers to help us access a memory. When we make a new memory, we include certain information about the situation that serve as clues to access the memory. A Retrieval Cue Failure happens when you are unable to recall a memory because none of the clues are present to trigger it.
For example, when you go shopping, you are often attended to by a certain lady at the checkout counter. Each time she rings up your purchase, you exchange greetings and engage in small talk. Even from afar, you always recognize her in her uniform at her assigned counter. But when you run into her at a coffee shop in your neighborhood, you are unable to recognize her because she is out of her uniform, in a different setting, and is a customer at the shop instead of behind a counter. In this case, none of the retrieval cues that go along with your memory of the checkout lady are present, and this makes you unable to recognize her (She probably doesn't recognize you either!).