Mnemonic devices are a type of literary device which helps you remember certain things. Mnemonic devices can be anything from acronyms, to phrases, short poems, etc. Like if you wanted to remember the order of taxonomy, Kingdom, Phylum, Class, Order, Family, Genus and Species, a mnemonic device would be something like:
Kids Prefer Chess Over Fried Green Spinach.
Hope this helped!
-Trumpular :)