byte
A byte is the unit a file's weight is counted in, the way a parcel is counted in grams.
In more detail
The figures grow in thousands. A thousand bytes make a kilobyte. A thousand kilobytes — a million bytes — make a megabyte, which is about what one photograph taken with a telephone weighs.
Weight matters for two reasons and no others: what it costs to send a file, over a line or by email, and what it costs to keep it, for the years an invoice has to be kept. Nothing about how the page looks depends on it.
What it does on a real document
A one-page PDF document written here weighs between six thousand and ninety thousand bytes. A hundred of them therefore take up about as much room as two photographs.
Most of that weight is the lettering. A font carries the drawing of every letter it can write, and a document that copies the whole of one carries thousands of drawings to print a few dozen words. Keeping only the letters actually printed is what takes 84.6 per cent off the weight of the documents published on this site, and one of them came out twelve times lighter than it was.
Where it comes from, and why
A byte has held one letter of the alphabet since the nineteen sixties, and the word has outlived every reason it was chosen for. It is now simply the notch every other measure of size is built from.