Watch the video or read the post. Or both. Whichever you prefer.
Sir Alan Gardiner was a British Egyptologist working during the first half of the 20th century; he made great strides in our understanding of the language. He wrote the lengthy Egyptian Grammar: Being an Introduction to the Study of Hieroglyphs - a book that is still one of the essentials on the bookshelves of Egyptian language students today.
One of the things Gardiner did was to sort and group the hieroglyphic signs.
He divided them into groups - twenty-six groups in total:
Each group was then assigned a letter of the alphabet, and each sign within a group was given a number.
So, the kneeling man (or 'seated man') - a hieroglyph you'll come across over and over - is the first sign in group A ('Man and his occupations'), and is therefore referred to as 'Gardiner's Sign List A1'.
This is an incredibly useful tool for Egyptologists to use when discussing the language, especially considering some hieroglyphs are almost identical to others - the birds can be a bit of a minefield, for example (click to see a larger version):
Gardiner's sign list is the standard now used in all Egyptian language books and tutorials. Whenever you see a hieroglyphic sign being referenced using a letter and number, it's Gardiner's list their using.
Wikipedia has a very useful set of pages dedicated to Gardiner's sign list here.
Coming up next: modern conventions for writing and pronouncing ancient Egyptian