Mnemonic comes from the Greek word mnemon which means mindful. Mnemonics today are systems of principles and formulas designed to assist or improve memory. These are all systems to aid memory.
Mnemonics are techniques and systems used to remember a wide variety of information.
When you hear this phrase you can be assured that a memory was cued. Something in your environment has caused you to remember something. When you originally remembered the thought, it was somehow connected to the cue. When the cue appears again the memory returns. There was an association made between the cue and the memory during the original encounter.