mnemonic phrase

 A mnemonic phrase is 12 to 24 word password, known as a seed.  If you have those 12 to 24 words you can recover your wallet and all of its transactions, all of its keys across multiple cryptocurrencies.  Even if you have used that wallet for years and have done multiple transactions, generated multiple addresses, across thousands of cryptocurrencies, all of that can be recovered from your one set of words.  These words can be expressed in a variety of languages.  They come from a specific dictionary, and this dictionary has 2,048 words in it.  Its a fairly small dictionary and was designed by Satoshi Labs, the company that first proposed this standard.  It was a development of an idea previously created by the authors of the Electrum Wallet.  The key idea that you have to understand is a mnemonic phrase made of 24 words is just a number.  Its a number that's written in English words, but it's still just a number.  The first word in this dictionary is ABOUT and last word ZEBRA and in-between there is 2,046 other words alphabetically, ABOUT, ABOVE, ARDVARK, etc. all the way down to ZEBRA.  ABOUT being 1 and ZEBRA being 2,048.  Now think of these 24 numbers in a row as a series of 24 numbers of 1 through 2,048, and you can begin to see how this is just a coding system. So when you write out a number in decimal you use 0 through 9 and the position of the digit is important, if you put lots of these digits together you come up with some pretty big numbers.  A bitcoin private key for example is a 77 digit long decimal number.  That can be encoded as 10 to the 77, because it's a combination of 0 through 9 repeated 77 times.  You can choose the first digit of 77, 0 through 9, then the second digit would be 10 more possible combinations, so 10x10x10x10... seventy seven times, and for 77 digits you multiple 10 with its self 77 times and you end up with 10 to the power of 77.  Now lets do that with the word of a mnemonic phrase.  The first word in your 24 mnemonic phrase (seed) is a number between 1 and 2,048, so 2,048 possible options for your first word, and 2,048 possibilities for your second word, you do that 24 times, and you end up with 2,048 to the power of 24.

Watch the video above for a further and a more detailed explanation.